The most reliable signs of end of life are a bulge somewhere on the tire, indicating serious carcass failure (rare), a cut more than ~3mm in the carcass that exposes the tube, or visible carcass through the rubber tread. The latter usually occurs after an inadvertent brake lock up and skid. All presage imminent sudden tire failure. Worn out tires produce damage by causing flats and crashes. Check the whole circumference of the tire periodically for cuts, thin areas and bulges. Rear tire wears 2x the rate of the front. Steve