https://www.millerwelds.com/files/owners-manuals/o2228r_mil.pdf
Here’s the manual for the machine.
I haven’t looked into this very deep but normally towards the end of a manual will be the electrical layout for the machine.
Look through that schematic until you find the word electrode and work.
Trace those wires back to the lugs that they’re connected to in the machine.
Attach your banana clips wires there for a reading of raw voltage.
It should show it on the diagram but a note here is that the work (colloquially called the ground clamp) is positive.
I haven’t delved into this machine s manual so I’m not too sure if it’s high frequency or not so I would look into that some first.
When I get back to a real computer I could take closer look at the wiring schematic.
And @Nathan welcome to The Forum