Place it directly after the claim you are questioning in your edit, then build the joke by adding a Sudden Suspense sting before it and a Huh reaction right after it to show the other person's confusion.
Über diesen Sound
This is a short voice clip of the Italian phrase 'Io non credo', meaning I don't believe it, spoken in just over a second. It lands as flat, incredulous disbelief, the kind of skeptical reaction that makes a wild claim sound even less convincing and turns doubt into a quick, dry punchline.