Tips on how to tell if someone’s gay?

Ok so I really like this guy that I’ve been talking to in classes. We just happened to sit next to each other at the start of the year and really hit it off.I can’t tell if he’s been flirting with me or he’s just generally a really nice person. He has had a girlfriend in the past but idk if that really means too much cos they weren’t really that close when they were together.He doesn’t have any social media so I can’t text him to go out or stalk his Instagram for clues (not like I’ve ever done that 👀). I’m really open about my homosexuality at school so he already knows about me.Ahh I’m just really confused and hope that u guys could help me out here. I just can’t tell whether he really likes me or not.