I always notice when MDS kicks in, but I've owned so many questionable vehicles in my past, that I have grown very sensitive to the 'feel' of the vehicle when I'm driving. It is not something that is 'in your face' noticeable.....but rather very subtle. I have also noticed it most often in city driving, as for the most part little engine power is required after you have reached cruising speed so the conditions are idea for MDS operation.
When in MDS mode, a slight increase in throttle will usually feel like a mild shutter, and a heavy press of the throttle will be followed by a noticeable lag or bog as it changes from MDS mode back to 8 cyl mode. Once you tune your senses to feel it, it is very distinctive.