How do you know if someone actually likes you or is just being nice?

I (21F) have never flirted with anyone or been anyone's crush (to my knowledge). I am also really bad at telling if people are flirting with me or just being nice.

Anyway, the guy I've had a crush on for the last two years had class with me my freshman fall and my junior spring of college. I started to notice (and then develop a crush on) him when he randomly sat beside me in class one day, which was odd because I usually sat alone and he usually sat with his teammates. It was a small not a tiny class (~40 people). We made small talk and that was it. He sat beside me one other time but didn’t really talk. The rest of that semester he would sit directly in front of me or at the table right beside me, and I would sometimes catch him looking at me.

During the last week of class, I decided to talk to him; I figured there was no harm in striking up a conversation especially with class ending. I noticed him lingering near me and my friends as we were walking out. We talked for a few minutes, found out we had a few things in common, and that was it. I followed him on Instagram after it popped up in my suggested, but he didn't follow back. I also didn't have any posts on my account at the time.

Last semester I had class with him again, but it was super tiny (it was like 10 of us). I started to notice that whenever we did presentations he only asked questions about mine and not anyone else's, and I would also catch him looking at me every now at then. At the end of the semester, I followed him again and he followed me back within a few hours. After a week or two I messaged him and thanked him for engaging with my presentations. He said he was glad we got to meet and that they were great. He liked my messages but didn't respond after that. He also recently watched my Instagram story, but it could’ve been accidental.

I think he was just being friendly, but I can't help but wonder if any of it was more than coincidental. Was he interested, or am I reading too much into it?