The best way is to learn as much as you can about the coins, and handle thousands of them. Unfortunately, this is not possible for most of us. Failing this, the best way would be to only buy from large, established dealers who guarantee authenticity, and have a business history so that you'll know they'll be around for a while. If you have a coin you're not sure of, there are authentication services (e.g. David Sear) and most of the large firms specializing in ancients will give an unofficial opinion, if you're a customer. They will also give a written opinion, but of course there is a charge for that.

Stay away from the modern coin grading services. With the boom in ancient coin collectors many of these services are starting to try to get into the market. They do not have the experience to identify, grade, or authenticate ancient coins. In fact, many coins "slabbed" by them end up being misattributed! If you look into their websites, they don't actually say they will authenticate the coin - just grade it. This means they aren't saying whether it's a forgery of not, they're just assigning a number value to say how nice of condition it's in. This is virtually worthless, and quite misleading, for ancient coins.

There ARE forgeries out there, but the problem is overblown. Numismatists of moderate experience can recognize the blatant fakes, and should have the experience to know warning signs of possible fakes, both by the type of sale, and the artistic and physical aspects of the coin.

Odenathus Sempronius
www.ruark.org/coins (non-commercial)