How do you know if a boy is gay/bisexual? And if he likes you?

So there is a boy in my last hour class, and his name is Zach. He is a Junior (16 years old) Some what built hunk, as he plays soccer (V), Caucasian male. I'm a Sophomore (in the closet), (15 years old), Twink like body, as I also play soccer (JV), Hispanic. I met him just this year (3 months ago) and he has always been on my mind. The first day I saw him, I looked at him and admired his physical appearance, but throughout the year I developed a crush on him.

Anyways during class when he would take out his writing utensils or flash drive, he would look at me for a while, I could see in the corner of my eye he would look at me. Then later, I turned to face him (to make sure), and I was right, but I quickly turned away. Though, I could still see him looking. Anyways his friend sits across from my table, and he goes there just to talk to him, but I always notice him look up to me then to his friends computer (constantly).

He's helped me on a couple of projects, because he eavesdrops on the conversations I have with my friend. When I asked my friend some questions he would answer them.

When I would have to present my projects, he would answer the questions I would ask to the class (He didn't with other projects).

Just yesterday I was walking out of Chemistry with my friend. I dropped her off at her class then headed back to my Algebra II class, and I saw him with his friend, he looked at me for a second, and kept walking for 2 seconds, then turned around again and took a quick glance at me.

Anyways he's never had a girlfriend (as far as I'm concerned), He doesn't go to homecoming, I didn't see him there. He's never had a homecoming date or any type of date. He seems straight when you talk to him, but that has nothing to do with his sexuality.

What should I do? I talk a few times everyday in class, he cracks a couple of jokes, which I laugh at, and he's a great guy.