If "he flirts alot" then there's yer answer..........you ain't the only one!
I never really flirted in my entire life really.. Well except once when my ex was wearing this T-shirt that said "Talk nerdy to me". So naturally I said she had a good graphic card. And that I really liked how her soundboard and everything was so wonderfully intergrated into her motherboard. Soo.. to be more helpful.
If you dont want to say anything, just look at him a lot. Was always obvious to me as a kid/teenager anyway. When girls just looked at me a lot, knew they liked me then and I would look back if I liked them. Although should be some obvious signs anyway. Excuses to see you or talk to you etc.
Ask him to help you with something. If he wants to and tries to, you know he does :)