Detecting a block under a block WITHOUT placing (naturally generated)

Hello!

So I wanted to make a dimension that had world generation like The End, and when I did, only one block was used to make up the floating islands, which is understandable. I choose grass as the block. I tried to bypass the single block type islands by making a procedure that detected if grass was under another grass block, it would turn to stone. But I failed, and I don't know how I would make a procedure like that. Does anyone know how to do such a thing? I'm new and don't know a whole lot.