I think my roommate let someone sleep in my bed when I was gone for the weekend!!!

So I went home for the weekend and the agreement between my roommate and I was that people may only sit on my bed if the bed is made (the comforter is covering the sheets). I got back today and she told me that one of our hall mates visiting friends had fallen asleep on top of my comforter and they couldn't wake him up so they just left him there.

Now I'm just getting ready for bed, and something isn't the same about the way the bed is made... but I can't pinpoint what's different. I'm really starting to think she lied to me and let someone sleep in my bed. Honestly, if she had been honest and said someone slept in my bed, I wouldn't care as much because I would know I have to change my sheets. But now I'm sitting here wondering whether or not someone did sleep in my bed, and I had to change the sheets because I just don't know.

So here's my question: 1. Should I say something to her, even though she's already claimed he slept on top of my comforter? 2. Any ideas about discrete little tricks I could use when I make the bed the next time I leave so as to be able to tell if someone did sleep in my bed, and then remade it?
I think my roommate let someone sleep in my bed when I was gone for the weekend!!!
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