Some tires have wear bars ... other than that, as a recreational rider, once I get one flat, when the tire is off it's easy to feel the tread thickness and make a decision as to whether there's enough left to keep it on, or replace it. If you start getting flats more often than usual that's a big clue.
Then there's dry rot and cracked sidewalls for tires that lie around more than they get ridden, but at 150 mpw you're not going to be seeing those.