It's too complex for measurement without the correct equipment. A multimeter isn't adequate.

You need to find someone who has an impedance bridge which generates a tone such as the Toa ZM-104 (rather old and analogue - a digital equivalent is more accurate), but at around £200.00 it's a tad expensive for one-off use.

I have one, which I use mainly for measuring 100v line circuits.

If the loudspeaker chassis isn't marked up, then you may get a fair approximation using a multimeter by measuring a known loudspeaker and comparing the reading for the unknown one with it... but it's rather hit and miss.

If a cab is 'switchable', it will have an auto transformer doing the 'switching', and it's possible that someone may have driven it too hard, causing the varnish on the windings to break down. I would guess that's the only problem you're likely to find other than an open circuit driver.