Know the coin or know the seller. If you do not know a coin is authentic using your own knowledge, buy from a seller that is reputable and has the expertise to authenticate it for you. This is the most fundamental advice for ancient coin collectors.
I will never buy a coin if I am uncertain of the authenticity, unless it is coming from a dealer with expertise that I trust AND they offer a guarantee without time limit. And I have been a dealer since 1997.