PTO switches can be ground switches or power switches
DC does not care where you put the switch
So at the PTO PLUG check that the pins show battery voltage between them when the switch is on and not when the switch is off.
For fun you can also check to see if the the power or ground wire is the one that is switched .
On the switch itself you are looking for changes in continuity
I generally do not bother to test the switch but jump the plug
If jumping the plug works & the switch doesn't then the switch is cactus