When you ask that, are you looking for good things we like in someone or traits which we can use as a litmus test to know if they'll turn out to be good people?
Wasn't sure, because I was thinking one thing and everyone was commenting the other thing.
I know these things are going to seem unpopular, but I often look for someone's body art (jewelry, tattoos, etc.), whether they do drugs, their vocabulary, their occupation/hobbies, and how complicated their lives are.
I am not discriminatory against any of these things, I know many great people who fall under all the categories and am guilty of violating a few of my criteria myself (you may be thinking of calling me a hypocrite in the replies, though this is less an ideology and more something that may work personally), but suppose you have a time crunch or a history of choosing the wrong people. Would you not consider possible covert giveaways? The first two will give a possible hint as to whether someone is "one with the crowd" while the third and fifth might hint at their thought process and the fourth how in-tune they are with their setting.