I assumed he'd estimated it based on how distorted the face appears behind the glasses. I do that all the time.

At this angle it's hard for me to do that, since I usually use the edges of the face to estimate it. negative glasses pull the line inwards, positive outwards. I can reliably tell when someone is wearing fake glasses (0 strength) for example, and probably estimate strength within 30% of the actual value.

If the image was higher res maybe I could estimate this case too. Or this professional optometrist is just a lot better at it than I am.

Strong negative glasses: (Note the faces contours in the glasses appearing well inside the faces contours around the glassed)

Fake glasses:

Positive glasses:

PS: Searching for generic terms yields 100% fake glasses, so I took a specific person I remember having strong glasses for myopia.