This is a tough one, all joking aside. How you play it depends on how confident and comfortable with yourself you are.
I'm an older guy but in decent shape and moderately successful, and I have passable game, although I'm no master.
I would probably just own it if caught, make eye contact and smile (sheepishly or confidently, but don't leer). Then maybe compliment her. Not "nice jugs" but maybe comment on the t shirt design or something she's wearing, or if you really want to go for it, tell her she's in great shape. There's a way to do it without coming off pervy, but it's hard to put into words. It's just a feeling and it's all about the vibe. And tune into her body language. Does she look at you or look down when she catches you looking? Probably a good sign. Does she look away to the side or up? Not so good. You can't generalize that she "wants you to look". Some girls dress just got themselves because they like the way they look, and some girls dress that way for attention. But you can't assume they want YOUR attention. That's why body language is important.
Then be ready to move on if she reacts badly. There are plenty more out there.