One of my phals that is growing in semi hydro had a single light brown raised circular blob on the underside of one leaf.
It seemed harder than the rest of the leaf.

I tried to rub it off with rubbing alcohol and it didn't come off. I ended up pulling the leaf off the plant and after "dissecting" it a bit it seems like the object is embedded in the leaf, not just on the surface.

What is the best way to tell if this is really scale, or if it is just a harmless scar from other causes?