In Internet slang, a troll is someone who posts inflammatory, extraneous, or off-topic messages in an online community, such as an online discussion forum, chat room, or blog, with the primary intent of provoking other users into a desired emotional response or of otherwise disrupting normal on-topic discussion
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.
Fiction has no limit
Non- Fiction can only reach Fiction
but how is this going to end?
Strygwyr, the Bloodseeker
"For the Flayed Twins, I seek blood."