For road bike use. rear tires will show the casing threads through the rubber when worn to (or a bit past) the replacement point.
Front tires basically never wear out as they are much more lightly loaded and don't absorb the driving forces. I've weighed front tires with 7000 miles on them and they weigh within 10 grams of their new weight and showed no tread wear. That doesn't mean they can be used forever. Aging, flexing, light exposure, etc. do damage the rubber and I replace the front tire every second rear tire.
Some riders "rotate" their tires by swithching them front to rear to even out the wear and replace both at one time. I prefer to keep the thicker tread tire up front the entire time.