Alien Detector

An Alien Detector is an electronic device that can supposedly tell if someone is an alien in disguise.

Bionic Bunny used one to detect that a little girl was really Kodo in disguise. To make certain Harry Mills was not an alien, Buster planned to use the device on him, but Harry accidentally broke it first by stepping on it. He had earlier tested it on the television, proving that a woman on a game show was an alien. Both Harry and Buster use the gadgets on Mr. Ratburn, and they both read that he is an alien.

Considering how the detector has never once said the person in question was not an alien, it is likely just a toy and doesn't really work.