Normally, grind holes are found on rocks much larger than the holes (see below). If your rock is in a very remote location, look for a mano. A mano is (usually) a rectangular shaped stone that has been worn smooth from the grinding. Some manos are round, but rectangular is most common (see below).
If you hit the area around the rock, and fine flour or flake gold, then yo0u will know it was used for ore grinding, most likely it was used by Indians for grinding food.
Best,
Mike