You could always try "I know this sounds like a chat-up line and I don't mean it to be, but". Of course sometimes that can just sound like a cheap get-out ...

Trouble is, that itself can be a chat-up line (and in fact one person who I had this rant at last night turned out to have successfully used it!).

In any case, as it turned out she wasn't the person she reminded me of, and in all probability wouldn't even have known her, so even asking her wouldn't have been helpful :-)