Martin walked past the window and incredulously saw that Sondelius was growing old.
Martin peered, incredulous.
Incredulously he slapped each of his pockets, and went the round and slapped them all over again.
He began, incredulously, to comprehend his freedom.
Incredulously he perceived women asserting that all Germans were baby-killers, universities barring the language of Heine, orchestras outlawing the music of Beethoven, professors in uniform bellowing at clerks, and the clerks never protesting.
Dr. Billy Smith whispered that he had gone in to see Tubbs and heard McGurk shouting at him, "Your job is to run this shop and not work for that land-stealing, four-flushing, playproducing son of evil, Pete Minnigen!" The morning after, when Martin ambled to his laboratory, he discovered a gasping, a muttering, a shaking in the corridors, and incredulously he heard: "Tubbs has resigned!"
There are no more uses of "incredulous" in the book.
Show samples from other sources
I find it incredulous that you believe she is sincere.