Look, you don't have to worry about the ones who are open about their activities. The ones with no self control. Those you can spot a mile away. They come off as charming and seems to offset their extreme views and actions. They show a complete lack of care for the consequences of their actions.
The ones you need to worry about are ones like me. The one who exudes charisma, confidence, and fakes emotion and sympathy to appear as normal and gain trust. These ones care about being caught. Not because being caught will expose them, but because being caught will hinder their efforts in the future. If they are to be caught they don't want it to be for something minor. If they are to be caught they want to go down for something big, then to have all their past decisions reveal and compound in the downfall. If they get caught with no notoriety, they've failed. (James Holmes, Anders Behring Breivik)
These types will assure you they are your friend, even go out of their way to prove it, if they find you useful. They will assure you, they only have eyes for you, having just fucked someone else then another after you. It's all about power over others and how useful you are to them.
Here's what I do that I guess you should look out for.
If they seem kind of more withdrawn from your group of friends, and you kinda have the feeling y'all are his only friends. Yet whenever there's a party here's asking about the girls and suddenly he becomes a social butterfly and life of the party, never has any problems getting girls.
The guy you met on Tinder says he wants an exclusive relationship, but he's really busy with school/work/whatever to give the time to the relationship you deserve. You only see him every 3-4 days and when you do you usually end up having sex. Those other days he's not seeing you, he's seeing other girls, and telling them the exact same thing he's told you. I even play off the "I'm a poor student" story so I very rarely have to take them out.
Very quickly climbs the ranks in work. Like faster than usual. Becomes a professional friend to the boss by studying their actions and behaviors, then modifying his own to gain favor while dropping subtle negative hints about other coworkers who pose a threat. Next thing you know, the guy that's been there just over a year, year and a half is getting that promotion you've been working hard for, and you've been there twice as long as him.
When people joke about hurting someone or killing them, they entertain the motion far longer than others would. Additionally, is known to be very smart and has a desire to join military infantry. Chances are, he wants training on how to kill people, and do so in a legal manner. Bonus points if he has an interest in forensics and politics.
They're able to get away and talk their way out of things a lot or all the time. They premeditate whatever they're going to do, knowing it's wrong and come up with a good, sound, defense in the chances they get caught or called out.