One thing I have always been curious about is exactly when to replace worn rims. The rim wear that most concerns me is the wear that occurs when the brake pad grinds against the rim, eventually making it convex. So far, each time one of my rims has failed, the consequence has been a rubbing brake causes be a rim edge that has splayed out.
This time the failure was catastrophic (without actually having any catastrophic consequences thankfully). Here is a picture of what the wheel looks like after an attempt to inflate the tire blew the rim right off the wheel.

Another way to look at the thing is this:

You can see how the rim was torn away from the wheel.

Here is a cutaway view, obviously the left rim is much thinner than the right one.
.

Finally, here is the best cutaway I could find for a new steel rim
.
All wheels have different dimensions, but the worn sidewall appears to be around .8 millimeters thick (I don't have an accurate device to measure this). The good side is about 1.2 millimeters.

This suggests a method for checking for rim wear. It starts with the exact dimensions of the wheel (so every wheel will be different). This is the diagram for a mavic open pro:

As you can see, the width is 21mm. With a micrometer, measure the width of the worn wheel at its narrowest point. The difference gives twice the amount of wear on each of the rim edges. My wheel appeared to wear by about .4 millimeters, so .8 millimeters difference would seem to be too much wear.