Really? I know you ride a recumbent, but even on that, I would have thought looking behind you enough to be sure of just how bright your rear light is would be rather counterproductive in terms of overall safety. Unless you ride very slowly, so you can afford to not look ahead as much.
A quick glance every now and then will tell you if it is going much dimmer than usual, but not if it is only reducing a bit.
I can see my rear light by looking down between my legs, and I will certainly check it pretty frequently to be sure it is still working, but I'm not going to see enough when doing that to detect a small reduction in brightness.
Just notice you edited to add "check our lights each ride", and I'm sure we all do that. I was talking about checking the brightness while you are actually riding (i.e. while in motion).
And now you have added even more, while I was typing