You have a huge crush on someone in your class. She makes you feel all warm and fuzzy inside. He has an big effect on your mood. They make you feel great, but you wonder whether or not they feel the same way. Believe it or not, your typical person (whether or not is a crush), has very similar emotions. Meaning, they are likely to feel the same thing that you'd feel in a given situation. If you happen to feel something when you hypothetically stare into your crush's eyes, because they're doing the same thing, they are also likely feeling the same thing. If you feel like there's a huge spark in attraction because you feel like you can connect with him/her in a romantic or sexual way, they are likely to be feeling the same thing. Their emotions are no different from yours, even if you don't believe so. They are human just like you, and as long as you see that you are exclusive to them, I can guarantee that your crush has genuine interest, general feelings for you.
His/Her behavior around you is different from everyone else's
This is all you need to see. If you have interacted with your crush and see this sign, put your insecurities to rest. You wonder why he's talking to everyone except you. You wonder why she seems to quickly break eye contact with you, yet can comfortably talk to anyone else in the room. You wonder why he talks to you less often, especially compared to other girls, yet shows blatant interest in you. If you ever see that your crush's behavior around you is very different from everyone else's, you know that you are exclusive to your crush, and that's all you could ever want.
Your given significant other and your typical lovers are exclusive to each other, and even though you see that he isn't able to talk to you as easily, you also see that his behavior around you is different. It's different because he cares about what he does around you. It's different because she wouldn't want to mess things up right in front of you, so she chooses to admire you from afar. It's different because they like you, and their actions around you matter to them. You are a beauty in their eyes, and you have been singled out over every other person they see; every other person they talk to, and that is truthfully the only sign you need to be able to tell if someone has genuine interest in you.
You still feel down since they can't talk to you as easily as other people and are confused because you believe they like you then, but now, you don't. Though if you see this sign in any crush, I can guarantee that he finds you just as beautiful, that she finds you just as attractive, that they find you just as great as you feel about them.