The truth spell was a spell used to force a subject to tell the truth no matter what.
Rupert Giles attempted to use this spell on Spike in order to force him to tell the Scooby Gang where The Initiative lab was. He chanted: “Enemy, enemy, be now quiet [...] Let your deceitful tongue… be broken. Let no untruths… be spoken.” However, he was not able to complete the spell because he had gone blind due to Willow Rosenberg’s "My Will Be Done" spell.
In order to interrogate the Vice President of the United States on the Pandora Project, Willow cast upon him a truth spell using the magic word “Veritas”. It made him act impulsively and relaxed, saying everything whatever came into his mind about what he was asked, while smiling drunkenly. However, a pre-existing spell prevented him to reveal who the leader of the organization was.