How to Find out if Your Ex Still Likes You?

what are some signs, (we broke up over a stupid arugement....nothing serious at all) and we both missed each other when we wanted to work things out. Lately he's just been flirty with me, and his best friend said he wanted to rekindle the spark between us. But he's the hang cliffer....he's been dating a girl for over a year (i haven't asked how she's doing, nor has he mention it....) But he flirts with me regardless he's in a relationship....so i'm not sure why he would do that....so now i'm asking for signs to see if he's looking to get back together or just being a guy and flirty.