Here is why I think Obama is a hardcore atheist: his ongoing macabre and stomach-turning support of partial birth abortion, and the strong stand he took for placing a baby who had survived partial birth abortion in with the dirty laundry, to die of hypothermia and dehydration. No man with even a scintilla of suspicion that a just God might possibly exist could champion such causes. Fear of judgment would override PC; the survival instinct is very strong.
Unfortunately, atheists don't have a monopoly on atrocities. Plenty of theists have supported things just as bad as the above, if not worse.