Is this something you could do manually and if so how?
@Trevor – if you want it accurate this can only be calculated by counting pixels in the rendered highres 1-bit image that is used for platesetting when all transparency is flattened, all color management, all the screening is done and the colors are separated.
If you want a well-informed guess, I would recommend the following steps:
1. Output to PDF with simulate overprinting
2. Render the PDF in PhotoShop to pixels
3. Evaluate the pixels with the statistic tools there*
* Don't ask me if this can be done by ExtendScript (but you could ask the folks in the PhotoShop Scripting Forum)