In Golden Sun: The Lost Age, Sheba learns the Reveal Psynergy after completing the test at Air's Rock. Maha, from Garoh, was also able to learn Reveal at Air's Rock, but due to his powers stemming from a different source, he was forever trapped in the form of a werewolf.
If you see any objects forming a hexagon of any sort, chances are, there's something in the middle that can only be seen via Reveal. Also, any hidden treasures will be sparkling when examined by Reveal.
A similar Psynergy, Search, appears in Golden Sun: Dark Dawn. Search is a Venus Psynergy unique to Himi; Reveal is not seen in the game, but it is mentioned.