When mogul sockets are manufactured, their ratings are stamped into the ceramics, or burned into the glaze when kilned. Sometimes its stamped onto the metal parts inside the socket. Your's should be easily identifiable.
A 6Kv 600Vac 2000Watt is usually standard.
A 5kv 600Vac 1500Watt rating is acceptable.
A 4Kv 600Vac is usually only manufactured for lower wattage fixtures.