Well, the way I see it, there are two ways you can go about this.
You can A) Be distrustful of everyone you meet, take extra precautions and NOT get screwed over faster than a nude Megan Fox in a ghetto...
OR, you can be put your trust in the people you deem trustworthy, and hope to god that your amazing judgment can't POSSIBLY be wrong.
Now, I rarely see instances of people from group A turning into group B, however I see plenty of occasions where those naive and gullible group B folks get fucked up the ass by that co-worker who just seemed oh-so-genuine. Human judgment isn't infallible; there are people out there who are smarter and cleverer than you, and if you are part of group B, then you better hope that they don't decide that they want something from you. They can, and they will outwit you.
Now, that isn't to say that EVERY human being has some ulterior motive. Some of them really are genuine buddies of yours, and you're lucky to have those kind of people in your life. The gamble in this situation is determining the genuine's from the fakes. And unlike on here when you want to figure out if that girl is REALLY a girl, these people aren't complete retards, and will probably play the part pretty well.