How to diagnose clutch master or slave?

[conesare2seconds]

Hi everyone. I could use some pointers, please, on diagnosing a clutch hydraulic problem.

I had the car out of the garage late this afternoon, replaced all the Bose amps. Went to take a short drive and found the clutch pedal was down and the take up point was at the floor. Holding the pedal down made it worse; I lost pedal trying to get the trans out of first and into reverse.

I left it in neutral and was able to get pedal back after a few seconds of idling in neutral. Pumping the pedal got enough action back to drop her in reverse to reposition the car then into first to put her back in the garage. It was nearly dark and I could get eyes on the clutch reservoir.

Is there a differential diagnosis process that will help me figure out whether the master or slave is bad? I'll move the ECU and check the reservoir. If the reservoir is empty, I think that'll point to a possible leaky slave. If full, I'd think bad master. Any advice would be received with gratitude.
Tom