Have one character's backpack full with items you are willing to throw away. The pack thus filled, walk around on the beach and hit the A button to search the ground.

If there is no hidden item, the character will just bend down and search the ground, but if there is an item, the character will say something about the knapsack being full.

When this happens, go into your backpack and get rid of an item, exit the backpack menu, and then tap the screen to dig in the sand. You'll find hidden items easily using this method.