Is it powdery mildew?

I have three girls (Soma) in flush, and my friend thinks they have PM. What I see on the leaves I have taken for older, prerhaps dried resin glands. (Yes, it looks like a light dusting, but I see nothing gray or fuzzy on any leaves or stems... no raised bumps on leaf surfaces, either.)

My question is this: Is there always damage to the underside of the leaf when PM is present on the upper surface? My leaves have no damage or marks to the underside whatsoever.

Is it powdery mildew?