A Truthsayer is anyone who can detect truth when it is being told, with the ability itself being referred to as truthsense.
Truthsaying was a skill developed and used by the Bene Gesserit Sisterhood. Indeed, the majority of Truthsayers employed during the time of the Faufreluches were Bene Gesserit adepts. Largely because of this skillset and the Bene Gesserit's ties to it, several factions viewed it as invaluable in gaining political power and prestige, which was implied by Feyd-Rautha Harkonnen when asking why his uncle, Baron Vladimir Harkonnen, never bought a Bene Gesserit as a sex slave, before his uncle while snapping at him implied the reason he never bought one was because of his homosexual tastes.
The ability to see truth was also seen as a gift that some individuals were born with. Paul Atreides was identified as one such individual by the Reverend Mother Gaius Helen Mohiam when she first met him on Caladan in 10191 AG.