You really can't, and that is usually the biggest problem. However, with time, you can learn certain indicators that can help identify a possible troll. I should note that one or two "odd" things does not mean someone is a troll. Truth is, some people have led unusual lives and sometimes life can be crazier than fiction. It is usually after an extended period, and after a significant number of "red flags", when you can truly suspect a troll.