If a guy usually flirts with everyone, he will probably be quite well known for possessing this characteristic!
That means a lot of your mutual friends will be able to attest to that fact. So first of all, try asking around, maybe not directly but indirectly, to get a sense of the type of guy he is. If it seems he is just a 'flirt', it is possible that he is just being himself around you, as he is with everyone else.
If you come to find that he is not generally known for being this way, there is a good chance that he is flirting with you because he likes you. Look out for other signs such as:
- eye contact
- body language
- subtle verbal language that may imply that he has more than just platonic feelings for you
These are all physical clues that hint that someone might have feelings for you. However, in order to know for sure, there's no way around simply asking - and seeing what response you get. If you're not keen on making the first move, why not try flirting back and seeing how he reacts.
If you manage to spend enough time alone together, I'm sure he'll make his feelings known fairly soon.
I hope this helps