Jeez, you guys talking about never knowing when a girl is interested in you are lucky. At least you've *had* a girl interested in you at some point, even though you didn't realize it at the time and probably slapped yourself silly when you did. It's far more depressing to think that you probably could tell when a girl is interested, but never getting the chance to find out because not a single one ever has been.
To date (no pun intended), I've still never had a girl show interest in me, at least in the terms of face-to-face interaction that you're speaking of. Online (which doesn't count in some ways and to many doesn't count at all) I've had two girls show interest, but for obvious reasons when it's online it has to be, well, obvious, and pretty much said through direct words. Although, it can be hard to tell that sometimes since over the internet a lot of the social standards that apply in face to face encounters don't apply. (For example, it's not uncommon for people to say they love you or <3 or something similar; so it actually is somewhat difficult to tell if they're being serious or not when they talk about how great you are.)
So, online, I would say the first move is defined very differently than it would be in your standard social setting, and that's the key point. First move depends *entirely* on the context, and the social nuances of whatever situation you're in. Since societal standards are often arbitrary and based on unspoken assumptions that are very hard to understand for most aspies, I don't think the 'first move' can be quantified to your satisfaction or with the precision or consistency that you require. The best I can do is tell you to give me a list of social parameters, the situation, the context, the activities that happened before it, and then describe the action in question, and then I'll tell you whether it was a 'first move' or not. Without accounting for all of those other variables, though, there's just no way to say.
I guess a loosely based definition of a first move would be anything that puts you outside of the comfort zone and/or is unusual for the level of relationship you currently have with the person you're making the move on for the social context that you're currently in, and that also indicates to them that you want to elevate your relationship to what is considered (either by society or by the person in question) to be a romantic and/or sexual one.
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Only once you have traversed the path of darkness will you come to truly appreciate the light.