I don't know how to describe it, it's just a feeling. If I feel weird about a situation I'll back out. It's always effective and it's never wrong. It's the only thing I trust to be honest.
An example: My friend invited me out to a bar with his group of friends. I didn't have id but he was insisting on me going and I said to him I don't have a good feeling about tonight, I'm not going. The next day comes around, didn't him and his friend get into a fight at the bar and two out of the four ended up in jail for the night.
Another example: I was talking to my good friend's mom about her daughter's new bf. She told me how he helps around the house, he cooks, he smiles a lot and super friendly. I told her to keep an eye on him because something doesn't sit right with me about him. Months past and the bf was stealing money out of the mom's purse, sneaking out of the house while on house arrest (that's another story) and drew out about $3200 out of his gf's bank account using the empty bank envelope scam. I knew he was bad news from the get go.