I would guess it's a lot like radiology. Look at an X-Ray sometime and figure how someone looks at that and can see cancer or fractures or whatever. My understanding of shadowgraph photography was that they were looking at the sonic cloud and making assumptions from the shape of the shockwave.
Nowadays, they have fast camera's and can take photos that really show bullet cant and yaw, in color, and slow-motion. I saw a timing system that would stop a bullet in color at <1mm intervals. That's fast mister. A blind man can see things in those photos, but, there was no price listed and I didn't bother to ask.
Thanks, I didn't know that(color shots). I understand about yaw etc. being in the photo, just wondering what reference is used to quantify it. Maybe with multiple exposures on the same paper your reference is down the middle?