Rogue PC keeps stealing my money

>rogue PC keeps stealing my money
What's an in character way to get him?

Before someone suggests, I talked with DM and he told me to solve it ingame. The problem is that I cannot accuse him because I haven't figured out it was him (I know it but that would be metagaming). I do know that my coins keep disappearing (by keeping a hidden note on paper, then counting), so what's a way of getting him in the act? And when I do, should I give him a warning or go to violence?

Punch his player in the face.

Treat your coins with contact poison, see who gets the symptoms.

>I talked with DM and he told me to solve it ingame.
Tell your GM that he is a colossal faggot, only he can't draw.

That would be actually interesting, any alchemy shop I take? I could go there and ask to buy in private, but he would probably find it suspect.

Still he wears gloves. I could wait him to scratch his mouth or any other sensible place, but I'm not sure he would roleplay that without I pointing this out.

Just rig your coinpurse to explode when opened, and store most of your money in the form of gemstones on your person.

Or use counterfeit/illusory money, like glamering a bunch of stones to look like coins. Then kill whoever you see trying to spend some stones.

What's your character, OP?
Just so I can calculate a hypothetical scenario and solution for you.

Rig my coinpurse? Seems good, but how exactly would I do it?

Counterfeit money seems good too, but I would have to ask our magic user for it and hope he doesn't spill the beans.

I'm a human paladin.

Store all your money in your anus. He can't pilfer your coonpurse if it's empty.

Good old mousetrap in the wallet!

That, or exploding runes on the coins themselves.