Is she interested in me? 'Salah' signal

What was shown in the video happened to me a couple of times. For all times, the girl I'm interested in is just being friendly (that's what you see in the video). But just like many other guys, I misinterpreted the friendly gesture and overestimated her interest - in fact, she's actually not interested in me! For many times, my friends has told me these signals are "a girl just being friendly".

How do I tell if a girl is genuinely interested in me? I've got into a crush before, and consulted Internet sources. They listed so many positive signs from a woman, but I find them not helpful.

The best observation I can do is to get into one myself! But who is the 'one' is a question I'm still attempting to answer....