It is not always easy just at first sight to determine whether someone is under the influence.
And it is very easy for us old hands to state that we would not fly with these characters if we suspected that they were under the influence.
However as young and impressionable PPL studes this would be more difficult to do.
If you do think your instructor is incapable refuse to fly with them and if necessary just take the coward's way out and feign an illness!
After that pluck up courage to report the matter to those in charge at the school/flying club.
FD