truth teller vs liar : go to Hell !
this problem seems to be the most faviourite of all, that's why it is posted here again and again, but anyways let me give the answer and spoil the fun for those who have not seen this before.
you ask the gaurd of one of the doors," what will the other gaurd say if i asked him, where does this door lead to."
whatever the answer he gives, the opposite is the correct answer.
figure it out yourselves.