You ask the guards: Which box would the other guard say holds the key?
Truth guy would tell you the truth and say that the liar guard would say the wrong box
The liar guard would tell lies and say that the truth guard would also say the wrong box
That leaves the other box as the box with the key They both say the wrong box holds the key, so the other must hold it.
(I don't know if my explanations make sense to everybody

they make sense to me)