open the fits file up in fits liberator and just hover the pointer around an area and you'll see values in the upper right pane...besides x and y you've got the pixel values, or ADU.
You can do the same in PI, but I think by default it uses normalised values just to confuse the issue.
If you register the images on top of each other you should be able to make some pretty accurate comparisons. Pick a favourite region of nebulosity, and some stars, and see what sort of values you get. That's it!