THE AIRPLANE LANDED GEAR UP DURING A COMMERCIAL AIR TAXI FLIGHT CHECK CONDUCTED BY AN FAA INSPECTOR. THE GEAR UP LANDING FOLLOWED A DISCUSSION IN THE TRAFFIC PATTERN OF THE EMERGENCY LANDING GEAR EXTENSION PROCEDURES. THE COMMERCIAL PILOT FLYING THE AIRPLANE REPORTED THAT HE PLACED THE GEAR LEVER IN THE DOWN POSITION, BUT FAILED TO CHECK FOR THE THREE GREEN LANDING GEAR DOWN LIGHTS WHICH INDICATE THE GEAR LOCKED DOWN. THE FAA INSPECTOR ALSO DOES NOT RECALL LOOKING AT THE LANDING GEAR INDICATION LIGHTS PRIOR TO LANDING. NEITHER PILOT REMEMBERS HEARING A GEAR WARNING HORN OR SEEING THE FLASHING GEAR NOT EXTENDED LIGHT IN THE LANDING GEAR SELECTOR HANDLE. POST ACCIDENT INSPECTION DISCLOSED THE GEAR LIGHT WAS OPERATING INTERMITTENTLY, AND THE GEAR WARNING HORN WOULD SOUND ONLY IF THE LEFT ENGINE THROTTLE WAS RETARDED TO THE STOP WITH CONSIDERABLE BACK PRESSURE. THE THREE GREEN GEAR LOCKED LIGHTS WORKED SATISFACTORILY.