The question is based on a flawed premise. I don't see how "they" (shouldn't that be "we"?) could "find out" Obama is an atheist unless he were to declare himself an atheist. That is, he'd have to come out. If he is a nonbeliever (and there are some serious people, like Jerry Coyne, who suspect that he is), then he has been skillfully and purposefully concealing his atheism for decades, presumably having recognized that his election to any major office would be impossible otherwise.
No one could out Obama. He's a prayerful churchgoer and by all appearances a steadfast Christian. He has his theism covered. If, however, something were to get him to change his mind and to deliberately reveal his irreligiosity and nonbelief, then he would be doing so for a significant personal reason, one that would certainly shitcan his political agenda along with the presidency and also do some serious short-term damage to the Democratic party.
In other words, if we were to find out that Obama is an atheist, the only plausible context (barring a mental breakdown) would doubtless also involve his resignation from office and from public life for profound personal reasons. If, in this respect, Obama actually has been living a lie, we will not learn the truth until he is long out of office and very likely not even then.
If he is a deceiptful atheist I doubt we will ever quite know, though perhaps he will want to "come clean" at a much later stage. I for one put the odds at 50/50 that he is one...
Presidents care deeply about the historical perception of their time in office, and a lie of this magnitude would forever tarnish his name. Best example would be Nixon.
If, however, something were to get him to change his mind and to deliberately reveal his irreligiosity and nonbelief,
When pigs fly. Or at least when you put lipstick on one.