The complement to the hiding skill, the ability to detect hidden can come in handy in many situations. At first the skill may appear useless as the magery spell Reveal would seem to serve the same purpose. However the skill is more effective in several ways. It requires no reagents, has no casting time, can be used while hidden, and has a larger range. The skill is activated from the skill list.
Having higher skill in detecting hidden also gives you a higher chance of passively detecting other players who are using the stealth skill near you. In addition, the detecting hidden skill can be used to locate faction traps if you are in a faction.
To train detecting hidden, simply use the skill repeatedly, targeting yourself.