All you can do is look out for red flags I suppose. Or even, yellow flags in a way. Some people may not exactly be 'evil' but, they may not be a good match in terms of friendship. They may be a 'fair weather friend' and be pretty unreliable. I suppose you have to know what you want from a friendship/ relationship and- if it looks like they won't be that person- it's whether you can accept them for who they are and accept whatever level of friendship you get or, whether you just shun the whole thing. I've learnt the hard way that it's so risky to rely on people. It's so hard not to build up expectations when you start to create patterns in your life though. But, 'evil' people? I doubt you'll know till you start interacting with them.