A "Rat" among friends. Need a good trap. What it do?

[BadMoFu]

To make a long story short, I suspect that one of the people I regulary hang out with is stealaing from me and others have this suspition too).

Once on a party in my place a $50 from my walet was stolen (most ppl at the party knew I had a big stack of bills in my wallet in my room, unwatched).

On another party at my friend's house his dad's wallet was stolen.

And I often have poker chips mistiriously dissapearing at my home.

So how can I trap that person?

I have a pretty badass plan by now, which I'll tell you later, for now I'd like to listen to your suggestions.