Even if the bike is the stolen one from the original poster, the new person riding it might not be the thief. No thief just wants to steal a bike and keep it. The thief would either just joy ride on it and dump, sell to someone quickly for money, or strip for parts. But yes use a mirror to read the serial number or use a cell phone to take a pic of it. If it is indeed the stolen bike then use a spare U lock to keep it on the rack and call police.