Originally Posted by FBinNY

The metal shavings you see in the pad are fairly common, and not by themselves an indicator of anything.

If the rim looks fine it probably is, but you can get a better idea on some rims because they have wear indicators. Not all these rims have these, but look for small, shallow holes or dimples, or a machined groove in the rims braking surface. If you have any of these, the rim is designed to be safe until worn ro where they disappear. Otherwise you can also get a sense of the wear bu feeling the sides of the brake track for a ridge where it's unworn. Rims vary but the wear allowance is typically 1-2mm, so there's quite a bit that can come off.

Otherwise, if you don't see anything, and there are no deep gouges which an embedded stone might have cut, odds greatly favor that the rim is fine. In any case, rims rarely fail suddenly due to track wear. There are usually early warning signs such as measurable bulging in areas which causes pulsed braking (dirt, or oil films can also cause pulsing), or hairline cracks forming at the line where the unsupported tire flange meets the braced part of the rim. If you see any of these cracks, or if the rim clicks or creaks when inflating tires failure is imminent.