Indigenous people used mano and metate to grind grain. The metate can be a separate vessel or a grinding slick in bedrock. Natural depressions, or gnamma pits, also occur in bedrock.  So how can you tell tell the difference between a natural weathering feature and a metate? 

If you found a mano next to the metate, you would have a solid piece of evidence.  However, no mano were found close to this metate. 

Without a matching mano, the next "test" to decide if a depression is natural or anthropogenic is roughness.  Mano grind and polish and smooth bedrock.  Weathering creates rough surfaces as mineral grains gradually erode.  Compare the smooth interior of the metate with the pitted surface of the surrounding rock. If the depression was due to natural weathering, the surface would be as pitted or more pitted than the adjacent weathered andesite.