We wrote a little tool to probe SMC and graph the output, so we know CPU temp and fan speeds and whatnot. If a fan were to fail, it shows up as 0 rpm speed (in my experience thus far), so we can tell and take the host offline.

Even if you can't see when the fan itself has failed, the CPU core temp should eventually go out of the acceptable range without any forced air at all, which is also helpful to determine that hardware maintenance is required.

So far nothing has actually failed on any of our Mac Pros though. When and if that happens, the entire Pro will get swapped out as one field replaceable unit, and then put in the repair queue.