First, the set-up: When you acquire the follower, the GM as usual determines loyalty in secret. Those with fanatic loyalty are definitely not secret supernatural emissaries, because fanatics would tell you. (I’m tempted to say even the +1 and +2 morale followers aren't emissaries, but I’ll let that pass.) For those who are less than fanatic boosters of your cause, the GM determines whether they act mysterious or ordinary, and rolls for alignment (I’d use a d6, 1 = Chaotic, 6 = Lawful, everything else is neutral.) Mysterious, non-neutral followers get 2d6-7 secret bonus levels of cleric ability: those with at least 1 secret level were sent by Heaven or Hell to join you for some unknown purpose. Half of your mysterious, non-neutral followers will just be creepy but ordinary.
So, you have a mysterious follower. Detect Evil might help, but might give a false positive. What you could do, though, is consult a sage or study religious/occult texts for clues identifying supernatural emissaries. Things like “infernal emissaries don’t cast a shadow by the light of a blessed candle.”
But even this information should not be fool-proof. The emissary might be clever enough to hide the telltale signs. The GM should use the lower of your Intelligence or Wisdom (or just Wisdom, if you consulted a sage … and the sage is accurate.) If this score is higher than the emissary’s Wisdom, or both Wisdom and Intelligence for infernal emissaries (they’re liars!) then you can automatically spot the signs when you look for them. Otherwise, you spot the signs on a 5+ on 1d6.
Written with StackEdit.