I've been crying on this subreddit about having been cheated on for quite some time now, I think I can answer your question using my own experience. There will probably be a lot of overlap in the points I'll be listing. Keep in mind that I was in a long-distance relationship for about five years.
She continuously ignores your requests. In my case, she really, really wanted to stay in touch with her ex-boyfriend. I normally don't have a problem with this, but he started flirting with her and even going as far as saying he'd still hook up with her if given the chance. I respectfully told her that maybe she should tone it down a bit, as she directed all her attention towards him (when chatting on Skype) and almost none towards me. She got pissed and ignored me all night, and probably talked to her ex-boyfriend instead. Something like this would repeat itself like five times throughout the course of our relationship despite me asking her to think about my feelings when something like that happened. When she was asked out on a date by some guy (who knew she and I were in a long-distance relationship) and she refused to decline his offer, I snapped and told her I needed some time on my own. When I tried to talk things over with her two weeks later, she refused to Skype me and acted like an asshole. That was when she was cheating on me with her private guitar teacher (some weed-smoking douche with nothing to offer. Said she only did it because he was attractive).
You're not allowed to do the slightest thing with other girls (like talk to them in class), whereas she is okay with things like modelling for some guy she doesn't even know and gets pissed off when you say it's not something you would do out of respect towards the relationship. I once voice-recorded one of my physics classes so she would know what it was like, and she ended up giving me shit for responding to a girl who asked what I was doing with my phone. She, however, would take pictures with random guys at concerts and dance with them even though they called her a bitch seconds earlier (happened at Eminem's concert in Wembley Stadium).
She does not care or show interest about things you are passionate about. I remember the Netherlands playing against Costa Rica during the World Cup in Brazil. I'm Dutch and football is a big thing here and she knew it. After the match, she would bitch about my country having won the match on both Facebook and Twitter for hours on end. I can name so many other examples... She did not care at all when I received the news that I graduated secondary school, she would get extremely offensive whenever I told her how awesome a certain concert was.
She does things behind your back and twists the story (not lying, just omitting information). It took me a while to realise she was actually using Tinder while we were together. The way she described it, it was something along the lines of Snapchat and COULD get his/her picture out in the open just for fun. I was too busy and distracted at the time to question it. Furthermore, she happily talks to other guys who have a history of slapping her ass or talking about her breasts in front of everyone else. Once you start asking her why she would want to talk to such disrespectful people, she'll get pissed off.
You start feeling like you are the one who has to ''keep her honour in check''. I swear to God, she'd keep encouraging people to ask her an anonymous question on Ask.fm while all she was getting were questions like ''how deep is your pussy'' and ''how can you fit those tits in those tight labcoats we have at school''. She probably can't help that, but she never really cared when people said the most vile things about her.
She sends revealing pictures early on, probably before you are even a thing. If you are actually dating and physically together, this would not be a red flag really. But I met her online, barely knew her and apparently, she was still working things out with the guy before me (who later came crawling back).
She will not tell her family that you are a couple.
Honestly, there are so many others things that I haven't mentioned yet. What hurts me the most is that she and I were together for a week last summer, I visited her (first time flying ever, on my own, connection flight and about 650 euros spent) and her if we could get into my bed for like five minutes just to find out what it's really like to be in bed together. She refused, instead we kissed and cuddled because that was all we had time for. Two months ago, I found out some guitar teacher took her to his house twice where they smoked multiple joints and got into bed together and did much, much more.
I've been hitting the gym like crazy ever since, it was all that kept me going. Thankfully I've had massive support from friends and family, and this one girl in uni seems to be into me. I'm falling for her, but taking it slow.