There's a script called "Magnum: the Edit Detector" (listed on this page) that will place a marker at every place where it detects an edit. The way that it works is that it looks for any abrupt change in the scene and places a marker there; it uses a frame-by-frame comparison to find these abrupt changes. I think that a flash might be enough to trigger the detection of such a change, if you have the sensitivity settings cranked way up. Even if Magnum doesn't work for you as-is, it gives an idea of the direction that you can go to do something similar---or even what script author you could contact.
The expression method underlying Magnum's functionality is the sampleImage method.