Place it immediately after the claim you want to question, as the punchline of a short reaction sequence, then follow it with Law & Order Dun Dun for a mock-serious verdict or DJ Airhorn Sound Effect when the boast deserves a bigger payoff.
About this sound
This four-second Tom and Jerry clip delivers the line 'Don't you believe it' in a brisk, theatrical voice with a teasing rise on 'believe'. It feels like playful, knowing doubt, sharp enough to puncture a boast without sounding angry. Taken from the classic cartoon about a cat and mouse, it carries a familiar nostalgic charm for viewers who grew up watching their slapstick chases.