I suspect that something has gone awry with the electric water heater.
We do get water that is plenty hot, but it doesn't seem to last as long as it used to. My suspicions are further aroused because one day the water heater breakers had tripped.
Is this indicative of one of the elements failing, or would that just give us lukewarm water?
2 days after asking this question, the breaker tripped again, and this time a reset did not get hot water flowing again.
I did the whole testing procedure on the elements and thermostats. Both elements were definitely bad, so I drained the tank (it helped to use the pump from my rain barrels), then went to the hardware store and found a full kit with 2 elements and thermostats for $32 USD, so I went ahead and replaced everything. I also bought the special wrench tool to remove the screw-in elements, as it was easier than finding a 1 1/2" socket and cheaper than buying a wrench that would fit.
All-in-all, a relatively fast and inexpensive repair.