1st - in 40 years of riding I've only worn out one chainring, and it was made of a soft aluminum. 2nd - I've never even heard of a steel chainring getting worn out, but I guess it's possible. 3rd - I'd say if all, or at least a great majority of the teeth are visibly worn down to nubs, then you can see that it's worn out. In other words, you can visibly see when it's worn out. 4th - back to the one chainring I did wear out (1st point above) - I could see that the teeth were becoming very worn and thought I'd keep riding it and see what happened (it was a 52 tooth ring, so I knew the chain wasn't going to start slipping over all the teeth at once as I pedaled). What happened is that eventually it started occasionally shifting to the small chainring on it's own. The "low" teeth (nubs) weren't able to hold the chain laterally and it slid off of them to the side, hence the self-shifting.