The dice penalty should apply even if the target is a Demon/Angel in Deep Cover.
---
Aetheric Resonance, found on page 184 of Demon: The Descent states that Demons are able to detect nearby Aether (and "aether-flavored" essence) expenditures. However, a cover is able to "seal in" the Aether - in other words, Demons cannot actively identify Demons or Angels while in cover unless they are expending their fuel.

Spoof, from page 112, says that a Demon can reflexively thwart supernatural scrunity, *even in a passive way*. As such, if a Demon is masquerading as a Whatsit, and the omission of a dice penalty would then imply the Demon is not a Whatsit, *the dice penalty would then have to apply*.

Furthermore, the Embed via which a Demon may spoof a Whatsit does not require spending Aether, *so Aetheric Resonance will not detect it*.

To come around full circle, though - using the Exploit Show of Power requires spending Aether.

So, let's break this down:

Demon Sue is using Deep Cover to investigate a hive of Whatsits. She meets Peter, who asks for her to prove she is a Whatsit. Sue suspects Peter is also using Deep Cover, so she decides to use Show of Power to demonstrate.

Now, there are two possibilities: either Peter is a Whatsit, and the dice penalty applies, or Peter has, through Deep Cover as a super-charged Spoofing, tricked reality into thinking he is a Whatsit. Ergo, since the lack of a penalty would betray him, the penalty must apply.

However, if Peter is also a Demon, after watching the Show of Power, he can spend Aether to feel for recent expenditures - in which case, he'd get a yes, and Sue would likely be the obvious culprit.

Therefore, Sue makes a convincing display, but Peter now knows she's not a Whatsit.

For a simple Demon detector, **trick them into spending Aether/Essence while secluded and then spend for Aetheric Resonance.**

---

Furthermore, giving a character knowledge falls into the idea of metagaming: characters cannot "see" their sheets. For example, Abed and Bob both try to grab the same sandwich. Abed gets to its first - in fact, he judges that about 11 people, slower than himself, could have grabbed it before Bob. This implies he has a higher base initiative - but, since he knows he and Bob have the same amount of Composure, he now knows he has more mental flexibility than Bob, as he eats the sandwich.

This is a preposterous idea.

As such, the idea of a character knowing whether a dice penalty is in effect falls into Metagaming.

Especially in a contested roll, the Demon would be able to feel a range of "I feel like I employed this Exploit really well, but he didn't buy it" to "That felt okay - not my best effort - but it seemed believable anyway."

Maybe the first was without the dice penalty, maybe the second was with the dice penalty - the Demon does not know; the player does.