Girls?

Some of the more obvious hints are them turning their body away from you, giving short or curt answers, avoiding eye contact, leaning back to get more personal space, closed or defensive stances (such as folded arms or tightly crossed legs) and their avoiding your touch.

Signs of interest can be found in open stances, playing with hair, touching you, closing the distance between the two of you, somewhat longer eye contact, looking at lips (often a sign of wanting to kiss), tilted head, imitation of your posture (especially if mirrored) and such.

Keep in mind though, those are signs of attention and/or interest. It doesn't mean they want to have sex with you (yet). It just means you're interesting enough to warrant talking to at this point. There's of course way more to it than this, I'll send you an article on it if you'd like.

If they're interested they'll do girlie things, worry about how they smell, look. But if they aren't then they will be paranoid, they'll have less responsive answers, be less open.

If they reply to your questions with some questions of their own to show the will to continue the conversation is one hint. Also getting their face hidden in a book, not looking at you after replying, putting on headphones, replying with short conversation stoppers, are hints of not being interested.

I don't know why it isn't obvious to you. Some girls respond to it in the same way. They try to avoid you, or they get upset with you very often, you feel that they get angry when you are around, they aren't that friendly around you most times, or they look at you with a poker/upset face (rolling eyes etc.) most of the times stuff like that. Some good amount of girls would do something along those lines if they don't like you hitting on them. It's not a "hard to get" game they are doing some times. Maybe if you see that happening more you'd understand the difference.

It's easy. If they don't seek for your attention or they don't talk to you first for a very long period of time then they are clearly not into you. Just try to take the hint or be straight forward and ask.