A lot of the time, I don't even know. I may just feel uncomfortable around someone and be wary of them. Then, they will do something and I'll realize I must have picked up on some nonverbal or behavioral cue. I'm just going to give instances where I've noticed something was off, or things that raised flags.
The tip off may be people with bad boundaries; people who encroach my boundaries or others boundaries even if done in a kindly manner. I just see ulterior motives.
I'm very wary of extremely gushy people who give tons of compliments and seem to just love everyone. Very few like this are genuine in their affection. Many I've encountered seem to be the type to act like they hate conflict and disharmony so much it pains them, but then they either jump right into it, exacerbating it by picking sides or actually create drama where there really isn't any. I usually see these people as taking advantage of a group by exploiting any conflict or creating conflict to exploit the group to boost their own social standing. As
@Lady Jolanda said, watch how they respond to being called on this.
And then people who laugh at others misfortune and suffering, or display little to no empathy.
Inconsistent people are suspicious to me. If your words do not align with your actions.
But then there are times when I don't pick up on things. These tend to be with the most dangerous types of people who can come off charming and moral. But looking back, there are always tells. And people do tell you who they are. And you have to really listen to that.
I just haven't always listened.
I think we all have our own issues. No one is totally good or bad... But I think obviously it's a good idea to be aware and avoid people who prey on others.