Given that Shadow to Light (check the sidebar) has lately been talking about atheist claims that theists are mentally ill, I propose a simple, fast way to check if a given atheist is mentally ill.
Have them pray.
You'd figure an atheist with no hang-ups could offer up a sincere-sounding, serious prayer to God thanking Him for their daily bread, requesting His assistance. After all, there's no need to believe God exists to do this.
But if an atheist would sooner choke than deliver a straight, calm, sincere prayer? That's probably a sign they've got some deep issues going on.