I have a "dead" Cal Spas C11GD Genesis (Balboa systems C11GD system label inside box).

It came with my house. Apparently it worked until 2 years ago, I think that a valve failed and permitted water to back up into the ozonator. This permitted water to leak onto a 2-speed pump (runs through the heater).

So I figure the ozonator and one pump are ruined (don't know for sure though). When I turn on the spa (by turning on the GFDI and breakers (there are 3 breakers(!); 1 in main box, a GFDI next to that, and one outside), nothing happens. No sounds, no indicator light, no action. PUshing the jets button does nothing. I vented the plumbing to make sure there was not "lock".

If I were to unplug all the pumps, heater, ozonator, etc., how could I test the board to see that it was functioning or at least not dead?

Make sure you turn the power off. Check the fuses inside the control box. May be one or two 20/30 amp fuses. You must remove them to check them for continuity. Check for proper voltage at the terminal block. If this doesn't work you need to check and make sure the board transformer is working. If it's working you probably need to replace the board.