This hobby makes no sense to normal people, and is
probably the least scientific of all observations, but with experience, comparisons and other tricks, some of us try to guess prescriptions by looking at:
The Cut-In effect
The Distortion Effect
Wearing patterns/wearer's comments.
If you'd like to try your hand, here are some pictures to help you study What They Actually Look Like. But remember in these days of high index lenses it gets harder and harder to tell.
Progressive lenses are possibly the easiest to spot due to the strange refelections they create at angles.
This page offers more insight into how vision is tested and corrected.
Here is a Snellen chart.
There are 10 lines on the chart. The third one up is the 20/20 line, you are supposed to be able to read this from the correct distance (for which see page). The top letter is the 20/400 line, which correlates to roughly -4.00. Links to fancier tests can be found on my How About You? page.
A panel of trusted obsessives look at stars' glasses, compare them to those they know and/or use the techniques above, and try to guess the prescription.