Lilo & Stitch: The Series S01 E14 Fibber

Pleakley's mother is badgering him about finding a wife, and has even found a perfect candidate on his home planet, so Lilo comes up with an idea: To tell his mom he is already engaged to a human. But Pleakley's family turn up on Earth, so Pleakley must put on a fake wedding, passing Nani as his bride-to-be. But Experiment #032 has turned up, Fibber, and will beep whenever someone is lying. So, as Pleakley tries to plan his fake wedding, Lilo and Stitch have to prevent Fibber from giving away the game.