THE AIRCRAFT HAD JUST BEEN REFUELED PRIOR TO DEPARTURE. THE PILOT SAID HE DID NOT CHECK THE FUEL CAPS FOR SECURITY. AFTER TAKEOFF, FUEL BEGAN VENTING FROM THE LEFT TANK. THE PILOT RETURNED FOR LANDING, BUT DID NOT COMPLETE HIS NORMAL PRELANDING CHECKLIST AND LANDED GEAR UP. AIRPORT PERSONAL FOUND THE LEFT FUEL CAP ON THE RUNWAY. POSTFLIGHT CHECKS SHOWED THAT THE LANDING GEAR SYSTEM WAS OPERATING CORRECTLY.