A way to attempt to flush out traitors is to confront them with their treachery. The problem is, this really only works if you're sure they're the traitor in the first place, which kind of obviates the need to flush them out in the first place.
A fun thing to do as a GM is to plant seeds of suspicion that someone in the group is a traitor. It's even more fun if nobody is actually a traitor.
Leia: Good luck, Luke. We're all counting on you.
Luke: Thanks. Traitor!
Luke: You're the traitor aren't you? You're sending us into a trap!
Leia: I have literally no idea where you're coming from, farm boy.
R2-D2: The farm?
Luke: What's with all the kissing then?
Leia: Look, we have no time for this, you idiot! There is no traitor! And even if there was, it makes no difference. This plan is our only hope. Get out there and fight!
Luke: How long were you with Darth Vader before he turned you? Or were you his agent even before then?
Leia: Trust me.
Luke: Dammit, there's gotta be a traitor here somewhere.
R2-D2: Why is everyone looking at me?!