A truthsayer was a device that was able to make the user extract the truth from anyone they wanted. A set of truthsayers fell under the hands of the Forge by 1945. During the transportation of them Joey Carlisle managed to steal one along with other alien technology. The Seventh Doctor, Ace and Hex tracked this one in order to remove the technology from Earth but Forge agent Merchant managed to get it back. (AUDIO: Causalities of War)

In 2015, a younger Lysandra Aristedes used one on Sally Morgan to gain the truth of why she was on the Forge train in Romania. (AUDIO: Project: Nirvana)

In an alternate timeline, Sally used one on Lysandra to get the codes to the matter dispersal weapon. (AUDIO: Gods and Monsters)