If you bought it from a local bike shop, they should have the serial number easily accessible with your receipt.
If you cannot prove the bicycle is yours and the guy calls the cops because you are attempting to take it back, then you can be charged with robbery.
Whatever route you take, it could get a bit unfriendly and it's good to have some friends with you.
Do you have any photos? A certain set of accessories, modifications, or scratches can convince a cop it is yours.
Notwithstanding many police officers' preference for a serial number, photos, distinguishing characteristics, a CSBR report, or a police report number have all been used successfully by people trying to get CPD assistance in recovering a stolen bike. July is high season for bike theft.
Did ya get it back?