Three easy suggestions: 1. Buy gold that has authentication features, such as the Gold Maple. (Check the radial lines. Check the privy mark with the date, observe the general appearance of the coin. IT VERY HARD TO REPLICATE SECURITY FEATURES 2. Weigh it WEIGHT IS ABLE TO BE REPLICATED ONLY IF OTHER DIMENSIONS ARE CHANGED 3. Make sure it fits in the appropriate Airtite coin holder. Compare to other coins of the same type to make sure thickness and diameter are the same. I think this is the simplest, and most overlooked way, as airtite sizes are known ahead of time. If you know how a 1 oz gold Maple fits in one, and suddenly your next one doesn't, one of them is clearly fake. UNABLE REPLICATE SIZE WITHOUT CHANGING WEIGHT I know there are many other ways to authenticate gold, such as acid tests or expensive equipment, these three methods together seem to be pretty simple and foolproof.