My GP4000's have wear indicators - two pencil-tip size depressions, the bottom of which is the end of usable tread. 2000 miles so far, and still fine.

I did have a piece of gravel go through one sidewall and tube at 800 miles. Then the next week a new tube blew through the small cut. I booted it with a dollar bill to get home, then with a strip of extra Velox rim tape - no problems since, and I should be able to just wear out the tire. You look for any distortion of the fabric casing in the damaged area - either its going to split out and fail, or it just keeps its shape and holds up.