Basically, friends are people who are left at the end, if that makes sense. Your friends are the people who after everything is done, are still with you, regardless of what you do, how you act, your weirdness, etc.
Honestly, though this could be a bit blunt, it kinda sounds to me your friends sorta have you around out of sympathy. I feel like that's what happened with my friend who seems to no longer be my friend now. He ended up sticking around me without actually wanting to, I guess sort of out of a sense of obligation. He was one of the people others would describe as a "good listener," that sorta thing. It's only recently that it's occurred to me that he didn't actually appreciate me, but was more doing it out of a sense of obligation, and saw hanging out with me as more of a "chore."
As for how you know, time, really. Time will unravel everything, and show you who's truly your friend, truly cares about you, and who doesn't.
As for what I said about your friends possibly just sympathizing with you, I mean, definitely consider the idea, but don't become hyper-suspicious of your friends, just keep in the back of your mind that they could just be entirely your friend out of a sense of sympathy.
Also, girls are kinda notorious for being vicious and socially manipulative and all that. They really like talking bad about other people behind their backs, rather than outright confrontation like men do. Men generally go more "I don't like you, I don't like your face, goodbye" whereas girls do this weird thing where they pretend to all be friends with eachother and secretly talk badly about other girls and it turns into a big rumor mill. Obviously guys do this to a small extent, but I feel like with girls, it's like many times exponentially worse.
But yeah, to me, it sounds like they're sorta including you just for the sake of including you, unless you can really figure out your reason WHY you're in the group, what they want out of you, etc. This isn't necessarily terrible, but it's sorta unfulfilling for the reasons you stated, as either you or them aren't going to care at all what's being talked about. So, over time, either you'll drift away from the whole group for not relating to them, or they'll drift away from you. OR, over time, the opposite will occur, some common ground will be found, and you might become more closer friends with just one or two of the girls from the large group, and you might have more of a "real" friendship.
Longwinded post, but I hope that clarifies things. I wish I could be less cynical, but unfortunately it seems reality is pretty cynical. Good luck.