The whenever I think up a riddle riddle

Since I gave the rights for the weekly riddle to Andrew, I cant ressurect it. I can, however, throw my best riddles at you. I will publish the next one at a time when I think up a new one. Be warned: These are my hardest.

A man was walking down a path when he came to a fork in the road. At the fork in the road where two brothers. Now, the man had walked this way before, and knew that one fork led to the Swamp of Illusion, where he would expeirience a painful death, while the other led to the City of Bliss. But, the brrothers had taken down the sign, and the man forgot which was which. Now, the man also knew the brothers. Onne always told the truth, the other always lied, but they could not be told apart. So, the man walked up to one, and asked one question. He then took one action, walked down the road he had choosen, and arrived at the city of bliss. What question and action did the man ask and take?

Hint: The man asked one of the brothers, who he did not know if that brother told truth or lies. Thus, the question must work for both brothers.