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Is there a way to test the brake light switch while not mounted on the master reservoir?

I hooked a meter up to the leads and took a small drill bit and gently pressed into the fluid hole but nothing happened. Didn’t press really hard as I was not sure if this would screw up a diaphragm or not.

You might damage the the switch if you force a probe into it. However If I recall it is a small hole. You could connect to your air compressor.
I haven't tested one to see how much pressure it takes to actuate one though.

You are right I hardly put any pressure on the drill bit as I was worried I would puncture the diaphragm.

I talked to someone today that said the same thing you did and suggested I try stepping up air pressure to about 100psi.

Then there is the issue of it being the old switch and who knows what gunk is up inside there.

Here is what I did.

It turns out the cheepo air blow off nozzle I have, if you unscrew the tip, the size and threads are the same as on the brake light pressure switch. So I screwed the switch into the handle of the blow off nozzle. Then I opened the air valve to the line and pressed the valve on the nozzle. I had the ohm meter plugged into the switch. The switch didn’t close until I got to 120 psi. Dose that seem like the correct pressure for the break switch to operate at?

That should be close enough. The pressure supplied by your foot would be in the 150 to 300 PSI range.