I'm no photographer, but it seems to me that using a photo won't work well. Considering tritium is only visible in low-light situations, it's going to be really hard to tell if the difference is due to the brightness of the tritium or the camera settings. Or just the overall quality of the camera.
A better way is to try to determine how old your sights are. The half-life of tritium is about 12 years; so in 12 years they should be about half as bright as when they were new.
If they're factory sights and the box came with a little manila envelope that holds the test-fire casings, often the date on the package will tell you when it was test-fired in the factory, which should be pretty close to its manufacture date.
The oldest set of tritium sights I had were made by Meprolight, and I started to notice them getting dim at around the 8 year mark. Even then, they were still definitely usable as night sights. I never found out how much more noticable the dimness became after that because I sold that handgun soon after.
EDIT: Oops, forgot to refresh my browser before answering; when I wrote this response there were no replies yet.