For the average collector, its all about trusting a reputable dealer. How you determine the criteria for that is up to you, but there are plenty out there. The best way to get started is to find a brick and mortar shop in your area... walk in, browse around and just talk to the guys to get a feel for everything. (Preferably when the shop isn't busy of course). If you prefer net shopping, try to avoid ebay when you start. There are many reputable dealers on ebay but you might not know who they are for sometime. I personally recommend www.vcoins.com which is a collection of many dealers selling their wares in one huge online catalog.
There are ways to have coins authenticated, but it sometimes can be more expensive than the coin itself
However, here is a list of professional condition graders and authenticators... http://rg.ancients.info/guide/slabs.html
Scroll to the very bottom for ancient coin specific groups.
David Sears' service... a prominent numismatic expert and author of many books on the subject, charges $35 for standard service, but it is not an actual guarantee of authenticity in his case.