If I test a speaker using a continuity tester I get a result of "resistance measured in ohms".
It's a continuity test performed with DC voltage from a test instrument. Continuity is measured as resistance not impedance.
Testing the terminals of a cab is not Testing impedance, that requires much more technical kit.
So you either know the impedance as its given on the cab, or you have a lab to check or you know the 'resistance' measured in ohms, "the ohmage".
You can then take "a guess at the impedance based on the resistance".