Nice, I give the following advice often, namely:

Very often, the cause of imposter syndrome is that there is, in fact, an imposter. It just ain't YOU.

Could you expand on this a bit?

It sounds like you're saying that someone else is making you (the receiver of the advice, not you, jrm4) feel incompetent. How are they doing so? Shifting the blame for their own incompetence? Trying to push work on you that really should be their work, but they can't do?

And, how would you detect this? By realizing that it shouldn't be your work? But that takes knowledge of how the organization should run that a person with imposter syndrome isn't likely to have. Even more, isn't likely to be confident that they have.