How do you know if you're in love?

I'm not dating him but we always kinda had a thing for each other and liked each other on and off for years and we almost dated a month or two ago. I know that this is a weird question since I'm not dating and I haven't even dated the guy but we been friends ever since he moved to my grandma's neighborhood and we hung out every summer I spent with my grandma and I ended up moving there about 2 years ago with my family, but me and him hit it off he asked me out and I denied him because I didn't want to ruin are friendship and I liked his old brother but then we started talking again and then stopped because we didn't have time for each other because of sports and family and the holidays. But over the years he has always been in my heart and he makes me happy even thought he makes me mad sometimes but what do you think?