Truth sense was an ability that was cultivated by the Jedi Order, which allowed the individual to tap into the Force in order to determine if another being was telling the truth. In general, a trained Jedi could quickly assess whether another individual was being truthful or not, but the technique did have it's limitations. For instance, if a being under investigation truly believed what they were saying was the truth, and they knew of no other knowledge that could contradict it, the Jedi that was using the truth sense would often come to the conclusion that the being was telling the truth. Darth Sidious used this power to easily divine that Nute Gunray was lying about the whereabouts of the traitor Hath Monchar.