So, how, then do we know where he's pointing?

You don't.

I just found out that the chief actually doesnt point at rare collectables, he/she always points out at the sea, but it doesnt mean its going to be there, so once the Chief does his magic and points, pause the game(Space Bar) and search the Island for a collectables, I also found that Coral collectables are found around the beach, the Tablets are around the rubble, the Turtle Shells are around the waterfall, but the Feather collectables appear randomly around the island....