Generally, the slop comes when the rubber pieces wear out or get compressed to the point that the bike "lurches" when accelerating or especially decelerating, because of the excess play between the sprocket carrier and the wheel.
A good test is to put the bike up on a swingarm stand so that you can rotate the rear wheel freely.
Put the bike in gear and try to turn the rear wheel.
A good cush drive will have little or almost no play in the rear wheel. (Some play is normal..how much really varies depending on the bike. My old FZ 750 had what I would call a bit too much play, even with new cush rubbers.)
A worn cush drive will allow you to turn the wheel back and forth more than an inch or two.
Best way to really tell is by riding at a moderate speed (20-30mph) on a level surface, and abruptly let off the throttle. Make sure the surface is clean and dry, no rocks or sand either, as the rear wheel will lurch a bit if the drive is ok or a lot if the drive is worn out.
I hope the descriptions I gave help as it is hard to convey it well by text.
And yes, replacing a worn cush drive will make the ride smoother, as long as the rider uses good clutch technique.