Personally, I wouldn't say a thing. She might be offended by it. But, if you feel you must say something, just say : Nice butt you got there. I don't think I would say a thing, looking is OK, but telling her could cause you a slap in the face.
how do i tell a girl i looked at her butt?
Write a napkin note: I was looking at a bike I want to buy, butt you were in the way. Sorry couldn't help it. Slap me on your way out if offended.