Run a few tests... at noon on a cloudless day. Theoretically, the ambient light won't change too much over the 20 minutes that you are running the tests. Frame up your subject as comparable as you can so you can simply roll through the images and compare them all at relatively the same point. So if you are taking a picture of a barn, compose it so that the entire barn is in the frame with the top of the barn at the top of the image. Then start off by hand holding... then go to the monopod, then go to a tripod with a wired release... or a 2 or 10 second delay. Then compare with and without IS. I have my money on the tripod without IS engaged... but that should be a given.
Also, sometimes you might be the deciding factor. If you are walking/hiking towards your subject, your heart may be beating to the point to where your entire body is shaking just a bit and while you may not notice it, it is recognizable in your shots.