A leak test should be pretty straight forward and not too expensive. They should have a meter that they move around all the hoses, connections and compressor that should signal if there is a significant leak. If none has ever been added and nothing has ever been done to it you may just need a good shot of r12 to clear the sight glass. Get whoever you take it to to show you how to look at the sight glass to determine low charge. Anyone should be willing to show you how to do that.