If the goal is to get the load of the entire cab, then just use the multimeter off the main output jack (or a patch cable plugged into same), yeah. If you want the rating of each individual driver (and they're not labelled) then you gotta open her up or pop the drivers out to get to the terminals (you will also need to disconnect them from the wiring in order to ensure that you are measuring just that driver not it's neighbors as well).
Basically the actual rated (nominal aka weighted average) Impedance (varies with Frequency) of the cab is the next 'standard' rating up (2, 4, 8, 16.... a few hifi speakers, mostly tweeters, but sometimes whole cabs, are 6, but most people treat these as 8... also a couple Traynor cabs - the 6x10 IIRC) because the meter measures only DC R(esistance) which is just the largest component of Impedance, but not all of it (voice coils are coils and therefore have inductance which is reactive; see above re varying with frequency).
As an example, an 8 Ohm cab may measure at anywhere from 4.x to 7.x Ohms DCR. Modern drivers tend to be in the middle of this range (6+/-); vintage ones are all over the place.