Osuche~ My mother saw that I had this sense early and fostered it, encouraging me to trust myself implicitly. The only time it has ever failed me is when I try to talk myself out of it but even then I still know.
I can't pinpoint one or even ten things that lead me to begin to construct my impression but I used to tell my dad to just give me 30 seconds on the phone with anyone he was dating and I could save him some time
My former boss still calls me to "feel people out" and I have looked over/read applications etc for Mr. Lil to help identify those with potential issues.
All this does not mean that I don't give people ample opportunity to prove me wrong. I always take it with a grain of salt, and continue on but I am possibly more cautious about how much I divulge.