A trolldar (or 'troll radar') is the feeling one has when they suspect they are being fed false information just to provoke a response (also known as 'trolling').
This acts much like a gaydar, in that it is totally intrinsic, and based on the interpretation of information one is given.
Steve: "Hey Larry, what do you think is the worst name ever?"
Larry: "It's got to be the name 'Steve'. What a stupid name. Anyone called Steve should be either killed, or made to eat their own shit!"
Steve: *suspects Larry is just trying to annoy him* "You're trolling me aren't you?"
Larry: "Yes I am. Good to see your trolldar is working"...