Step 1: Has the ''fallacy'' been outlined by an antitheist
Argumentum ad hominem - that's a fallacy, to be clear. Either it's a fallacy or it's not, who has cited it is irrelevant.
Step 2: Google the ''fallacy''. If the first entries direct you to sites like RationalWiki you know were on dodgy ground.
Guilt by association - that's a fallacy, to be clear. Where you find the explanation of the fallacy doesn't matter, unless the formulation is incorrect.
O.