oo ooo I know the answer to this one.
I dont remember the logical reasoning behind it though, nor the exact question, but it is something like this:
If I ask your brother about which road will take me to the town, wich direction will he give me?
Lets assume road A leads to the town, and road B to the desert.
If he is asking the lying brother, the lying brother will say "he will tell you to use the B road" (his brother is telling the truth and would point to the A road, but lying brother is lying, therefore falsely stating that brother will be pointing to the B road)
If he is asking the truthful brother, the truthful brother will say "he will tell you to use the B road" (his brother is lying and would point to road B, and since truthful brother is telling the truth about lying brother, he therefore truthfully states that lying brother will be pointing to the B road)
Therefore, ask that question, and chose the other road than the one you are being pointed to.