Of two doors, one leads to freedom, the other to danger

Of two doors, one leads to freedom, the other to danger. A guard stands at each door — one always tells the truth, the other always lies, but you do not know which is which. You may ask only one guard a single question. Meet thinks that without knowing who the liar is, there is no way to be certain of the right door. His sister says there is one clever question that always works. Who is right?