for me I just know.
Ask him lol
Can you feel it, if you have face to face chats and relax in their company, then you will know. If not, you will as you get older
If they give you cold cash or presents.
Flirt with them? See what they do
I guess I'm too old.... I don't play games anymore. If I want to know if someone likes me I'd ask them out, or ask them if they're already in a relationship or some question that pertained to our getting to know each other better. If they say no, not a big deal.