Originally Posted by
chasm54
If it's a treaded tyre, loss of tread. If not, obvious signs of wear, flattish patches. In extreme cases, visible thread casing. Recurrent punctures can be an indicator, while you have the tyre off the bike check how thin it has become.
Yes, all true, and certain brand/models have specific indicators like the two depressed dots on Continental Grand Prix 4000. When the dots disappear (tire is worn level with the bottom of the dots), Conti says it is time to replace. I have never gone that far, Recurrent flats have always encouraged me to change the tire sooner. Of course, YMMV.