Pre World War 2 Gold Pendant

I want to buy a gold finish cross pendant but how do I really know how old it is?The seller said: "This very ornate, old, pre world war 2 cross pendant is marked on the side 1/20 12k G. F.. Also marked J & S. 1 1/2" high by 1" wide. Excellent condition. Has hanging loop" Please let me know ASAP !!! Thank You