They have the natural knowledge attributed to all Angels present in creation. It is diminished as not all is was shared before the fall, their speculative knowledge from revelations is intact. The knowledge of charity etc. they are devoid.
I.e. They know a lot.
Thier ability to deceive, if one grants them said ultimate deception, then extends into corruption of speculative reason. As we all know Reason's doctrine which includes the law of causality and ex nihilo nihil fit, collapses under its own contradictions already. Manipulation of memory to further corrupt conclusions derived from speculative reason are child's play.
Anything short of Revelation is corruptable.
Typically the "inability to detect" demons and indecernable ultimate deception I think really should be ignored, because otherwise the game goes off the rails and becomes all about Catholic theology.