I will just seize opportunity. Once, a guy approached me and compliment my Markiplier shirt, and left without asking me for my number. I ended up seeing him minutes later, went up to him, pointed it out to him and just asked for it.
I don't necessarily wait for specific signals, I just go when I feel the opportunity is best.
I approach totally on gut feeling. I don't looking for an obvious physical sign of interest that says "come get me, big daddy!".
I don't need a signal to talk to her initially. I just go and start a conversation based on the scene, and if i FEEL that she MAY be into me while we're talking, then I make a move to flirt a bit and to get her contact info.
For me, attraction is 100% intellectual. I can't find a man physically attractive unless I'm already into his personality. I wait until a mutual interest comes up, and use that as an ice breaker to form the basis for a pre-relationship friendship. I prefer getting to know a person before dating them. A lot of people swear against friends before lovers, but think about it: this eliminates the awkward "nothing in common" the occurs so often after the "honeymoon phase" is over. Too many couples form over superficial things like sex, money, or physical appearance these days. Those things may be nice bonuses, but they do not hold you together at all.
Me approaching a guy? Puhlease, I would never. I have no spine for that. Now, if I knew the guy and liked him, I would just be talkative and joke around a lot with him. If he liked me, he would see "Ohhh, she's liking our conversation... noice"
And if he didn't, lol, I would never be with him because I legit have no spine for that.
As long as it's not straight out obvious someone likes me I don't approach people because I'm seriously oblivious when it comes to things like this and in those cases they usually approach me first anyway.