I’m certain that my roommate has been stealing money from me.

I noticed that I was drastically low on money like two weeks ago, but I credited it to the fact that I had been spending a lot on food. I went with my friends this past Saturday and I made sure to count exactly how much I had and how much I had spent. When I got back to my dorm, I put my wallet in its usual place. My roommate was up at this time, but she went to bed shortly after. I went to bed to because I’m in the band and we had a game the next day. *time skip to this morning* I’m at my house and I open my wallet and there’s only $20. Keep in mind that I hadn’t needed my wallet this whole time, so I didn’t have a reason to open it and check to see if anything was missing. How do I confront her, and how do I avoid this in the future?