I would suggest "Quit checking me out
" Whether or not she is actually checking you out, doesn't really matter. Its the fact that you said it and if she is, she'll respond positively or shyly.
Honestly, it takes a certain amount of confidence to throw lines like this out to random women, but you have to start somewhere. If you have no problems with confidence, then just go out there and throw whatever bullshit comes to mind because if you are really comfortable, then the woman will feel that and respond positively regardless. If you aren't that comfortable, maybe it will benefit you to improve your inner game. there are lots of threads here to help you with that. I personally started with "How to be an alpha male" ebook. Check the link in my sig.