This gift allows you to determine when those around you are trying to deceive so you can react accordingly.
To use truth sensing you must have true motives. If your motives become even the slightest bit tainted, then you will start to perceive slightly tainted truth, which further taints your motives, which taints your perceptions, which taints your motives, which taints your perceptions, and on and on. It becomes a trap from which it never ends. This is why you MUST be honest and have true motives in developing and using truth sensing.
All people have the innate ability to sense truth, but being able to develop the gift of sensing truth is like having a second dose of this ability.