Well, I supposed that if an atheist went every Sunday to a place of worship, you might suspect they're not really atheists. But then by now there are support groups and organisations for PRIESTS and MINISTERS who stopped believing in god, and not just go to church but actually prepare and perform the service. So that's not very conclusive.
In the end, I personally think that what might actually cast a doubt over whether they've actually internalized atheism, is favouring one religion and discriminating against others, without any good evidence and good argument as to why. Like if anyone consistently hates Jesus, but consistently runs to be the first in line to plant his nose into Allah's butt, you might start to wonder if they actually are equally just against gods as a whole, or is it all just a butthurt reaction.