Posted this in a reply to another thread/subreddit. Figured people could discuss/expound/disagree here.

"What would a person who actually wanted to be approached be doing? "

Usually follows this order except #2 and #3 can be interchanged:

1) Checking you out/being okay with you checking them out. People naturally make eye contact/scan their environment, if someone finds you interesting they will make eye contact more than a few times (ideally it wants to be you catching them looking at you, not you looking at them and they look back to see who is creepin). Always look out for any NEGATIVE facial ticks when you make direct eye contact with them: better/neutral signs would be them smiling, staring with a soft or lost look on their face, or neutrally staring back. After accidental eye contact or them confirming that you were checking them out, they will either a) avoid all eye contact with you (which is a sign of no) or b) look back again at you to see if you're still looking. There should be some kind of "eye contact game" that ensues -- if she stops playing its over.

2) Trying to grab your attention

This is when their behavior changes/doesn't fit the situation relative to what they were doing before. This can be the hardest to decipher as it is up to the girl/her personality to decide how obvious/subtle she wants to be. The range is extremely wide, but most typical body language/seduction reads apply here but the key thing is that she is doing it for YOU, if there are other possible stimuli that could cause her to do the following, assume it wasn't you: Changing her hair style, raising her volume when she speaks/higher (more girly) tone, appearing less "busy" then before, doing something physical that could attract your attention, etc etc.

3) Increasing proximity or finding a way to be around you when there are less people:I think this is the clearest sign for approach. Girls could be doing #1 or #2 because they LIKE attention or are looking for some validation but may not want to actually engage in conversation. If AFTER she has done #1 with you, at the very least she should assume that you are on some level interested/attracted to her. As you can see in other posts, most girls will absolutely avoid being alone/near someone they find "creepy" aka someone overtly interested in them. If they feel safe/intrigued enough to go near you AFTER you have established non-verbally you've noticed her, then thats your sign to go in for the approach. A more concrete example is if she sits next to you (remember if #1 was not established first this means nothing), follows you around, chooses a seat/space far away from other people but plenty enough space where two strangers could fit without encroaching on each others personal space. The harder move to spot is called the "hover" where they make their way near by you and pause "looking for something" and then walk away. This gives plausible deniability on their part.

Feel free to add stuff I've missed or if there are alternative experiences on the matter.