Okay, on your Original question ....
Tell tale signs NGA usually involve being "
interested in you", which is basically the best gauge unless you happen to be someone blessed with so much
good looks people just can't get past appreciating your features. Now if you happen to be the average to above average dude and you want to know if a lady likes you back, then normally you'll still have to squeeze it outta her.
You have to determine a few things the most important of which is: Does she like your company? .... Which is NOT something you own ha.

But rather 'what you have to offer'. Which is basically the reason why the mating ritual usually fires off via a date invitation.
Beyond that, you have to gauge how she reacts to your presence. Does she enjoy the first time she sees you in the day? Will she miss your company when it's time to call it a date? Reading women's reaction, their eyes, isn't really very difficult. But then of course first, you've got to know women, at least a bit to be able to understand them.
If you've got a good sense of humor, then it's BIG plus. Reading women gets easier during unguarded moments and I personally think during the times they laugh they're easiest to read. .... And making them laugh goes beyond "knock, knock" Jokes ha. ... Sorry.

I'm sure you know that already.

And I tell you, even the typical 'dumb chick' beyond 23 years old gets more interested the more you make them think .... which once again goes beyond asking them crossword puzzle answers .... but again, I'm sure you already know that too.

All in all, there's really no fool-proof gauge, especially one that is universally applicable. Every woman is different in the same way that every guy has his own style.
Now if the question you really have in mind is: "
How to Make sure you Won't Get REJECTED", then the answer to that would depend on two major things:
1)
Your Target Woman - Her personality, her likes, dislikes, her ideal guy, among other things
2)
YOU - What you have to offer, your personality, your capabilities, your level of attractiveness, your ability to be interesting, among other things