My test is two fold. The 'feel' of the pad, usually close to the center of it, and the second is the 'look'. I find with 2000 and 4000's I get a very light discoloration on them after 5-7 uses. I use plenty of water, a small nylon brush to take off any particles between pads, but the 2000 and 4000's seem to 'wear down' a bit faster than the other grits.
When I do plug work, I start with a sandpaper product to cut the larger parts, followed by the pads. I will, be it right or wrong, after a 2000 doesn't seem to be 'normal', write on the pad a 3000+ and use this for any small minor cover adjustment that someone may want. If there's no use for it after a while, I use the foam portion to apply a polish, when needed.
Once I feel they are no good for either, I use the pad to 'cushion' the ball that is waiting to be picked up by a customer, between the ball and cup, just extra cushion. As you can see, I'm cheap and will use them for things long after the intended use is gone. I live in a dusty area and will use the foam portion to dust off the shop items, as needed.
The lower grits ( 180, 360, 500 ) seem to wear faster than others and that's the reason I went to sandpaper for those grits. As I said, maybe wrong, maybe be right but never had any complaints, and in these times, gotta' save where ya' can.
--------------------
az guy aka: R & L Bowlers Pro
rlbowlerspro@cox.net