According to Ignoramus5533 :

I have a related question. I want to sell them on ebay and would like
to get as much as I can. The obvious things I can do is wipe the dust,
photograph them very well and test them with an ohm meter. Is there
anything else, something unobvious, that I could do?

Check for cracks, corrosion or pitting on the connectors. Ensure
that the contacts open/close when you operate the handle.

If you can see/get at the contacts, check them for heavy pitting.
[Sometimes these things have replaceable contacts, so you may
be able to get at them.]

Beyond that, you'd need to test them to see if they trip for
overcurrent. This is absolutely _not_ something you can test
short of having a purpose built lab with a lot of very
expensive gear, least of which being a power supply that can
deliver one heck of a lot of amps. And a shorting switch
that won't explode at, say, 100,000 amps.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.