When buying ancient coins, the MOST important point to remember is weight. So never buy an expensive gold coin that has no weight stated. If its not weighed dont even consider it. Weights can vary slightly when the coin is clipped or heavily worn, however significant differences would cause concern. Secondly, look closely at the flan or flat part of the coin with an eye glass. If there's minor pitting, sought of small depressions all over, this would indicate a cast fake, typically from Turkey and the fakes can be very good and can be lower denominations of fairly cheap coins. The casting process leaves these small pits all over and cannot be removed as the air bubbles leave the slowly cooling metal. 

Good Celtic & Greek gold fakes tend to be the more abstract or the very common die pairings as in the case of Atrebates and the Celtic horse, we have also seen crude Ambiani. All would appear to originate from the South Coast around West Sussex and East Anglia. All appear as classic Celtic horses, all off flan, all a bit too crude. One characteristic which is common are the file marks on the edges, look very close under an eye glass and check for very faint file marks which conceal the casting pip. An ancient coin, 2000+ years old, would have smooth edges.

Also remember gold was precious way back then and forgers did copy coins in antiquity, beware of copper sheathed gold staters, these do exist and can be mistaken for the real thing, see point about weight above.

As we move into uncertain times (Jan 2007) dont be fooled into thinking there will be good coins to be bought cheaply, occasionally you might have a real win, but generally, any ancient coin offered without weight or any real provenance, ie, "it came from me diggin on a mates field with a metal detector and I dont know what it is" should be treated with suspicision. Most if not all detectorists are well versed in ancient artifacts, sadly some use this as a means to introduce fake coins into the market.

AND finally, dont buy coins abroad unless from proper dealers and certainly not from tourist destinations. We have seen, many fake Greek As and Roman 'bronze' sestertius that have been bought for $10 each, again from Turkey, Spain, Morroco and around the Med, that are all cast fakes.