We ofte talk about sign of interest in girls we meet at the club or somewhere during day time for the first time, however it would be interesting to try to see which sign are shown to us from girl we meet often that somehow want us to make a move.

1)Do the phonecall to plan the group activity could be a sign, difficulty a girl would bother call or text someone she doesnt like at all (except if he is the only guy driving or is useful)

2)Offer to do the buy togheter could be an other one, like asking you to join her to go buy the drinks alone, hardly a girl like to stay alone with a guy she doesnt like.

3)talking about sexual topics, friendzoned guys usually are emotional tampoon but never get the dirty side of the chick.

4)Allowing some touch here and there in intimate parts

Many more could be suggested from you, only problem is that many signs of interest from girls can be misunderstood in case of a bad friendzone (which we avoided by making sure she doesnt see us a chumps).

It's common especially at school to be unable to understand that some girl has a crush for us only to found out years later...

Good question, sometimes we don't see what's right in front of us.

I can think of a few things, this is assuming that you've haven't dated the girl before, are not real close friends, and have no other previous "history":
1) She initiates most of your conversations with her.
1a) If she has your number she'll call or text out of the blue just to see what you're up to.
1b) If you can't ever walk past her without her dropping what she's doing to talk to you then she definitely wants your attention.
2) She's always quick to say yes to any group events that you invite her along on. (Annoyingly however she may still turn you down on a date: which likely means that she only wants to be friends though it could also mean that she isn't comfortable enough to be alone with you yet.)
3) She stalks your Facebook profile. This is a rather weak one though as some chicks just follow everything that their friends do so don't get your hopes up just because she's clicking "like" on everything that you say. On the other hand if you get the one who let's it slip that she's been following you silently (recently I had one reveal her presence when I posted something that she took offense to, until then I no indicators that she was paying attention to anything I wrote.)
4) She already has a boyfriend but never mentions him, even if you know that she has one she'll never bring him up unless you ask her about him and even then she'll hardly tell you a thing.
5) If she gets out of a relationship and feels the need to inform you that she's single again, but doesn't talk about her ex at all. (Had this one happen to me recently, I had known that she had been single for a couple of weeks but never commented or acknowledged until one night we're talking and she just decides to mention it in passing. Still it wasn't exactly subtle.)
6) As for physical contact, does she give you the "I don't really want to touch you but want to be polite" sort of half-assed hug or does she grasp on like she doesn't want to let go? If you give her a high-five and grasp her hand right afterwards does she pull away immediately or hold on to you for a few seconds? How does she respond to a playful shove to the lower part of her back when you want to direct her somewhere?

That's all I got for now.

That's all bull. The only interest sign you need is a yes to a date or her grabbing your meat, everything else is mental mastrubation when it comes to actual looking for signs. Strong interest is very obvious!

There is a fine line between friendliness and flirtation.

If she is always around you, starts conversation, and small talk, seems nervous around you, etc. then she's interested. She wants YOU to make the move following that, as women are indirect.