Here's a big list of questions. Recommended: best done over glasses of wine with your SO. Alternative: print out the questions, go off separately to write down your answers, then come together to discuss when you're done. Can be more honest.
Probably should come with a warning: if you find yourselves disagreeing more often than not, tread carefully going forward. If you both dive into the issues, discuss sensibly and work out things relatively smoothly, it's a very good sign (and congratulations).
The Questions:
Kids -- whether to have them at all, if yes, how many? When? Like after certain milestones (finishing school, established in a career, etc.). Husband gets vasectomy or wife gets hysterectomy after X number of kids?
How will you prevent unplanned pregnancy? What to do if an unplanned pregnancy occurs?
Will one of you be a stay-at-home parent? Can you afford that?
If the fetus is diagnosed with some defect or disfigurement or disability, what will you do?
What if you can't have babies? Do it artificially? Adopt?
If you're feeling really adventurous, bring up paternity testing (warning: this will probably nuke the relationship). However, some very brave, loyal and open-minded women have said they demand it, so their husbands will never have doubts.
Money -- Who pays for what, major purchases to plan and save for (house and cars), how to budget, debts, credit scores and credit histories. Limit on price before needing to consult each other? Like $100. Or have a joint account for shared expenses and separate "discretionary" accounts for buying what you want? Is your partner a saver or spender? (This is a BIG red flag). Getting a pre-nup? Bonus points if she suggests it first.
Sex -- kinks, deal-breakers, turn-ons, turn-offs, what you want to try together, frequency (e.g. Is once a day a lot, or not nearly enough?). If you're really brave, dive into sex history and numbers of past partners (warning: once you hear "the number" you cannot un-hear it, and vice versa when she hears yours). Threesome? FFM? MMF? Must be friends (more trustworthy) or must be strangers (no emotional attachment)?
How to respectfully reject sexual advances when not in the mood? Offer a consolation gesture, e.g. oral? Or a simple policy like if she rejects you twice in a row, it's up to her to make the next sexual advance if she's in the mood (or vice versa if the guy has lower libido). What to do if one partner's libido and sexual desire later plummets? Agree in advance to get hormone levels checked, seek treatment, etc.?
Relationship -- What constitutes cheating? Policies on opposite-gender friends? (if you're straight). Yes, it's okay to have guy/girl friends, but no, you're not allowed to sleep in the same bed with them.
Open or closed relationship? How to handle contact from ex-boyfriends/girlfriends? What happens if one of you gets flirted with when you're out together (or on their own)? Example: If a guy is hitting on her, it's her job to ward him off. I let her handle it unless she shoots me the "Rescue me!" eyes. Then I swoop in and scoop her away to safety. Clarify expectations.
Health -- Do you both work out, diet, cook, etc.? Any mental or emotional problems? Any chronic physical conditions that could hinder her? Any recurring ailments from her family's side that might show up in our kids?
Education and Career -- any big commitments that partners will have to work around.
Is one of you moving away for school? Who works while the other studies?
Will one or both of you be working insane hours at the start, e.g. medical residency or first year at an investment bank or high-powered law firm? How will you make time for each other?
Location -- where will you live? If you both want to get out, do you agree on which place you'll move to? Does one of you want to move while the other wants to stay home, close to family and friends? That can be tough.
What if one of you gets a great job offer in another town? Do we move or stay?
Household tasks -- Who cleans what? Who fixes what? How to respectfully tell each other to do something? Example: Don't just say, "The garbage can looks full" and expect the partner to do something. Say, "Can you please take out the garbage? It's getting full."
Religion -- Which one (if at all), and how devout you'll be. Which religion will the children be raised in?
Politics -- Do you both broadly agree on which party to support? Agree on the big issues?
Sports --What teams do you support?
Wedding -- Do it at all? Big ceremony or small ceremony? What's your partner's definition of big or small? For you, it might mean your parents and one best friend, she might think small is 100 guests.
Who pays and what's the total budget? How much to spend on a ring? Where to have a honeymoon? Who's invited and not invited?
Boundaries for bachelor and bachelorette parties? (Do yourselves a favor and ban strippers for both).
In-laws -- Do you get along with your partner's in-laws and vice versa? How often will you see each other's families? What's their health like? Is there any chance her parents or yours might move in with you?
What's her mother like? What's her relationship with her father like?
Friends -- Same as in-laws. Does your partner have friends you cannot stand? That you think are a bad influence? Can you handle them being in your partner's life and affecting your relationship? What's the quality of their relationships? It could impact your relationship.
Like are all your partner's friends single/divorced and partying, or married and stable? Do they sabotage and undermine you with your partner behind your back?
Or are they supportive and happy that your partner found a good match in you? (Although it's a red flag if their friends tell you stuff like, "You're so great! All her/his exes were terrible!").
If you don't know any of your partner's friends because he/she keeps them away from you, it could be a red flag of trying to hide the past. Preventing you from hearing things like, "Oh man, back in college [partner's name] did so much crazy stuff!" Examples: promiscuity, substance abuse, etc.
Affection -- What are your love languages? Do you prefer to be silent and communicate through thoughtful actions and practical things like filling her car with gas? Does she prefer to hear verbal affirmations and be physically touched often?
Would she be more thrilled if you left sweet little notes in her purse? Would he be happy if she brought out a can of beer to him in the garage when he's hot and sweaty from working on his car?
Arguments -- What are your arguing styles? Do small things bother you? Or do you shrug things off and save your battles for bigger issues? Does she tell you things immediately, or let them build up until she unloads the past six months of things you've done wrong?
Do you prefer to walk away, calm down and think before engaging? Does she need to hash it out right away? Neither is necessarily better, but you need to consider each other.
Do you act like adversaries? Or do you act as a team against the problem? "It's not you vs. me, it's you and me vs. the problem."
Personal space -- For example, when you arrive home from work, do you want to "ramp down" with TV or music for an hour before talking? Does she want to talk about her day as soon as she's through the door? When you're stressed, do you like to be comforted or left alone?
Do you build in "guy time" to hang out with your buddies without her? Same goes for her to go out with her friends. Schedule in some time apart to release pressure with friends.
You had lives before you met and those experiences attracted you to each other. Don't let those lives and friends slip away.
Exception for toxic people; cut them out as soon as you can. Examples: Deadbeat Dan who always wants to borrow money or Negative Nancy who always complains about her boyfriends and insists to your SO that her relationship with you will fail too.
Travel -- Now, before kids, when you retire, or never? I highly recommend before kids. You can always take family trips later, but you won't have the same vibe as pre-kids (sex on the beach). What places are on your "bucket lists"? How about experiences, e.g. kung fu class in China, digging up artifacts in South America, a candlelit dinner in Paris, cruises in the Caribbean, etc.? This is a fun topic, so I saved this for last so you guys can end on an upbeat note.
It's worth mentioning that some churches offer "pre-marital counseling." I'm not particularly religious, but I saw this mentioned by another redditor and I thought it was a fantastic and practical idea. I think it can help to have a third party bring up the issues. Makes the discussion less adversarial (you vs. your SO) and more the both of you against the issues, e.g. "Father Damien made a great point about schools. We should think about that when choosing where to buy a home."
The redditor said in her program, they had group discussions with other engaged couples going through the same thing, and also got to speak to experienced older married couples for frank advice. I'd check with your local house of worship if they offer programs like that. Even if you discover you're not compatible and decide to go separate ways, on the positive side it's better to find out now than through marriage counseling for divorce later.
Getting back on track, how does your partner make you feel when you have conversations like this? Do you feel comfortable being open and honest? Does your partner? It's okay to be a bit nervous going in. But if you feel a lot of tension, shame and embarrassment throughout the whole conversation, your gut may be giving you a warning of deeper incompatibility.
On the bright side, do you both laugh in relief and think, "Thank God we agree on those things! My SO is so cool"?