Coins found in a genuine archaeological context (ie with real archaeologists doing the excavating) should be OK (though again, there was that Time Team
episode where someone planted a fake ancient coin in the trench overnight, where the team found it the next day... caused a bit of a stir until the guy owned up to it).
The easiest way to spot a fake is to find an exact match on a fake coin database like the one over on FORVM
. Absence from the list is not proof of authenticity, but presence on the list certainly is reason to doubt.
It's difficult to definitively declare "fake" or "genuine", for all but the most obvious cases, based just on pictures. Show the coins in person to just about any expert in ancients, either dealer or collector, and they'll probably be able to form a better opinion.
If don't have any ancients experts nearby, there are a couple of "third parties" you could send it to. ICG is the only American third party grading company to routinely slab ancients (though you should be aware that most ancients collectors loathe slabs). And David Sear is the author of the Greek and Roman catalogues - he runs an authentication service, and provides historic background for the coin. Their services are expensive, so you'd only do this for a coin that was worth the cost.
And note that even in person, a good fake can fool the experts.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis