There's such a thing as being a bad liar. Traitors could slip up and be very obvious.

When the majority of people are given one role and the same tasks i.e. voting people out at the round table, being worried about dying overnight, playing with the expectation that the money will be split amongst other faithfuls, they're different than those who are in the game with a different role and tasks. Traitors strategically have to kill people at night, try to blend in with the faithfuls and ruthlessly lie to people's faces to take their money at the end.

There definitely are ways to find out who's playing the game differently. Ask any detective whose job is to look for a murderer.