Are you talking about scam prevention during the trade? Use a trusted third party escrow.
Gold coin would have to be sent to escrow to verify the validity of the gold coin, otherwise the problem would be what
xetsr just mentioned. This also implies that escrow would have to be knowledgeable in gold coins.
You can test for real gold by measuring dimensions and the weight, but to be really sure you'll have to use an XRF. These are not cheap, and most coin shops or local dealers should have one. If a local dealer does not have a XRF machine, then you'll know to never buy there.