How do you know if you really like someone?

This is definitely a tough question, simply because each person has a different idea of what they want when looking for a boyfriend/girlfriend/partner or whatever. It also changes dramatically as you mature mentally and emotionally.
I'm assuming you are in high school, and well, back then, my idea of liking somebody came down to enjoying the time spent with them, that they looked cute, etc.
things completely changed once I went to university. There you start thinking about the goals you have, and if these goals are compatible with that person you liked.
for example, I once dated this really nice girl that was not only beautiful but also incredibly smart. However, she was the home type that had never been outside the usa, and for her, traveling was out of her mind. Me on the other hand, I love to travel. As much as we liked each other, I wasn't willing to compromise this part of my personality to be with her, so I moved on.
so things are not as simply as saying, 'oh, you know ull like somebody by the feelings, etc' that all depends by the person. Right now your outlook of life is completely different of what it will be when you're 21; however, as you meet guys and are on the chase, what maybe you need to keep in mind is, what aspects of a guy's personality you liked or disliked, and as you get older, it will help you get a better idea of the guy you really like.
sure, there are some lucky people out there that meet somebody and they know that they have met the right person. However, now that I've been in the real world for 10 years, the more I have met married couples or those in long term relationships, the more I hear, 'oh, I was looking for this type of person and then he/she came along. ' so the keep on chasing, but be aware of your likes and dislikes! :d

Sounds like, what you initially feel is lust, which is completely natural! Liking someone for longer. Normally means love.
as you begin to get to know the guy. It might be that you find he isn't what you hoped for and he's not really your type.
so you come to the point where yeah, sure you were attracted to him. But lust doesn't last for long, and you didn't love him. So you grew more and more disinterested.

make any sense? Hope it helps! :)

Well you know when you like a person when he's the only person no your mind, when your talking to your friends and his name comes out,In class or in the halls you want to touch him , you love looking at the back of his head, and you know him no matter what he's wearing.

You'll know when you don't have to question what you feel. You'll just know that you like him. I used to be like that. I'd 'like' someone until they liked me and then poof, I'd get over it. But then I met a guy and I just knew in my heart I liked him. It's like you can't breathe, you can't stop smiling, the sound of his voice makes your heart race, you want to see him all the time. I can't really describe it but it's intense.