(Ignoring the miscellaneous supernatural detection abilities that might be in non-Demon related books, like Mage.)
Whenever a demon is subjected to an effect that might reveal her to be a supernatural being, she may attempt to “spoof” the effect, fooling it into thinking she’s an ordinary human. A demon cannot spoof in demonic form, for obvious reasons. The demon does not have to be aware of the effect to spoof it; spoofing happens reflexively. The demon does not necessarily know where the incoming detection attempt is coming from.
Spoofing doesn’t affect any power or effect that doesn’t explicitly detect whether a target is human or supernatural, nor does it inhibit any effects other than that detection.
The thing is, there are only two effects in the whole book that seem to be able to detect supernatural beings/demons in particular: the Supernatural Merits Aura Reading and Unseen Sense (God-Machine), which stigmatics and mortals can have.
There are several mentions of spoofing that imply there are other methods, but they don't explain further at all.
Like the Flagged Condition:
Flagged
The God-Machine or one of its agents is suspicious of you. It doesn’t know for sure that you’re a rogue angel, but it’s got you marked as a potential problem. Attempts to spoof agents of the God-Machine suffer a –3 penalty.
Resolution: Convince an agent of the God-Machine that you’re an ordinary human without spoofing it.
And the Blown Condition:
Blown (Persistent)
the only way to lose it is to destroy the compromised cover. You can’t spoof angels, God-Machine cultists, or anyone with the Unseen Sense (God-Machine) Merit.
But I can't find a Numen that an angel could even have that would identify a supernatural being in the first place. The mention of "agents of the God-Machine" and "God-Machine cultists" could be mortals that have Aura Reading, but there doesn't seem to be anything an angel can ever do that could be spoofed.
What can an angel do that a demon can spoof?
Weirder is the Behind the Curtain Exploit description:
Demons can use the Spoof ability when entering Infrastructure and are well-advised to do so in this case. Spoofing is described on p. 112. Successfully Spoofing adds two dice to this Exploit’s roll.
Spoofing isn't something you can do of your own initiative, by definition it's always in reaction to something, but I can't find anything in the book that says there's something like an active effect of 'scan for demons' at every Infrastructure entrance.
