How to know for sure a girl likes you?

Ok I liked this girl in 7th grade, we used to talk a lot back then but of course she didn't know I liked her, this was 2 years ago so now we freshman going on sophmores. We always like speak in the halls and every now and then conversate but anyway 2 days ago we was excempt for taking our finals and i was just minding my own business and talking with a friend and she came up to me and asked do i want to watch a movie with her on Netflix on her phone. At first i was surprised because usually she be hanging out with her friends but i just yes. We went toward the back of the room and sat down and watched the movie on her iphone. When ever a good part of the movie comes she seemed to get closer to me( btw the movie was called never back down). She always smiles at me durin the movie and ill smiled back and we both laughed but I really know if she does like me or not. I mean I try read up on the internet about if a girl likes you but i got a lot of results that really didn't help and i don't want to make no assumptions please help.

It sounds like she likes you. Whether or not she wants to go out with you is another question, but for now I would definitely say that you should return the favour by inviting her to do something with you. Don't look too far ahead, just take it step by step.

She's obviously happy to be seen in public as just the two of you. On the surface that seems like a good thing, but the public nature of what she's done raises another possibility. If she's a manipulative person then she may have done this in front of another girl who likes you to try to hurt her. If she's genuine then she's shown a really nice level of confidence that you shouldn't take for granted.