My roommate is stealing from me. How do I kick her out?

She's not on the lease. She's just paying me rent with my own money. I know if there's no contract I can kick her out but I'm worried about if she'll get a lawyer. I don't know what to do. It wasn't that she was taking the money - it wasn't that much - but now I just don't trust her and she's a good friend. So how do I catch her and get her to fess up? What are my optioins?