I never got scientific. In my case, I would haunt gold and silver shops and pawn shops for years. Those guys know what they are doing, and I never found something priced wrong. When I started looking at ancient collectables, I would find dealers where the numismatic value would almost be at the price of bullion. This happens, or used to happen, sometimes with St Gaudens double eagles priced the same as a bullion eagle. I also recalled that happening with gold 40 franc Napoleons.