Is there a way to measure (to determine what shot size was used) by looking at or measuring the part that was shot blasted? Trying to determine whether 330 or 550 was used.

First answer: Maybe
Second answer: Maybe not.

Here is an approach I would take if I had access to some unpeened material. I would peen a small area with S-330 and then a different area with S-550 (keeping machine parameters constant). By having just a few dents the dent size could be measured for comparison.
(Now for the Maybe Not) However: If the peening is done at the same INTENSITY (adjusting machine parameters) the diameter of the dents may be virtually the same. This knowledge comes from some work done several years ago by an engine manufacture. They were exploring methods of non-destructive inspection of peened surfaces. If the machine settings were kept constant (air pressure, media flow rate, etc) then a difference in dent size for the two media might be detected.
Another technique you might consider is to test the surface roughness with a profilometer. Get a profile of surface known to be peened properly and then test the unknown.
Good luck with your forensics.