Is there a way to test whether the coolant reservoir cap is working
properly? I think mine failed and caused a coolant leak (I can't find
any other possible source of the leak). I have a 98 SL2 with a
pressurized reservoir. I replaced the cap two years ago. Even then, I
wasn't quite sure it was the right thing to do (apparently it was, as I
didn't have any more problems...until now).

It's called pressure testing the cap and any shop can do it cheap.
The trick with Saturns is that they have to be very tight for some
reason or the will steam leak. Good luck.