Preach it!
In my (fairly successful) dating career I was almost always the aggressor (as it turned out, because almost all my boyfriends were of the “nice guy” type who didn’t want to look like assholes by flirting). What a turn-on when a guy openly and unapologetically expressed interest that was actually recognizable as such.
If you approach it in a light-hearted kind of way (as in “Hey, you’re cute, wanna go out?” rather than “I love you and want to be your white knight and you’ll break my heart if you reject me, there I said it, what are you going to do about it?”) then the worst that can happen is that she’ll say No thanks. Most likely she’ll be quite flattered anyway.
How to ask if we’re “attached” - don’t.
I find it really insulting when people only take “no” for an answer when it comes in the form of “No, thanks, I’m attached.” What’s wrong with just “no”? Besides, in this age of diversity in romantic relationships and sexual preferences, a “yes” doesn’t mean I’m unavailable any more than a “no” means I am available.
I kind of resent that MY “no” isn’t sufficient, but the implied “no” from my husband (who isn’t even present) is.