How do you know if your child has worms?

I ask because today when we were getting DD1 ready for her bath we noticed an enormous number of scratch marks on her bottom cheeks. She was at childcare today so we didn't see her itching all day, but obviously she must have been. Is it likely this scratching might be a sign? (There's only so far she can reach into her nappy..). If not, any other ideas? I'll be watching her more tomorrow when she's home.....