Reveal

Reveal is a Jupiter psynergy that allows you to see objects not normally seen to the naked eye.

In Golden Sun, Master Hama teaches Ivan the Reveal psynergy in order for Isaac and the others to survive through the Lamakan Desert by seeking out hidden oasis's along the way.

In Golden Sun: The Lost Age, Sheba learns the Reveal psynergy after completing the test at Air's Rock.

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.