If you're collecting gold coins or investing in them, there is a fine opportunity that earlier or later you will be introduced to a uncertain sample that you would question about whether it's a actual article or a counterfeit. It's not simple at all to identify false gold coins and you may not mark them at first (and perhaps even at last also, but you must identify at least a few things that might aid you not to fall into a trap.
The oldest method to ensure if the coin is actual was simply freezing it - please note, I don't suggest doing it, but rather just state it. You must have seen it in movies about older times, fortune hunters, and pirates. It may appear strange but in fact, there is a very balanced justification for it. Gold is extremely flexible metal, comparing to a number of base metals, such as copper or bronze for instance, and biting a actual gold coin would leave some teeth marks or at least leave a reaction that it's bitable