Ding, ding, ding... BDD is correct.

You ask either of them, "What would the other one say if you asked him "Which path is the correct one?"?" and you take the opposite way.

Say the correct path out is to the left...

If you ask the above question to the one telling the truth, he would say the liar would say "Go to the right."

If you ask the above question to the liar, he would lie and say the one telling the truth would say "Go to the right".

Therefore, it doesn't matter who you ask. Go Left, Young Man.