Ask guard B
Answer yes or no. Would he (guard A), "tell me that this door" (the door behind guard B) leads to freedom.....
Guard B will ask guard A....if guard A is the liar, he will tell guard B opposite of the truth....and Guard B will answer to you truthfully....if guard A tells the truth, guard B will lie to you....
So do opposite of whatever they say.
I think I typed that right...
Later EddieO