Riddle: Win One Rep by Answering this Riddle

[Aoi Magi]

I would ask both:

"If I asked you which door leads to the check, which one would you point to?"

Wouldn't work as they can both show their own doors. You have to ask each guard what the other one's reply would be to which door is the correct one.

Basically

D1 - G1 - tells lie
D2 - G2 - tells truth

Assume D1 is the correct door

Ask each guard which door the other guard will show:

In case of G1(liar): G2 will point to D1 hence G1 will lie and point to D2
In case of G2(honest): Will correctly say that G1 will point to D2.

So you just have to do the opposite to both and enter D1.

This only works if you assume both guards know of each other's honesty or dishonesty.