I think you'd need to pop the speakers out of the cab and measure them (ohm meter) or at least do the 9volt battery test on them each in turn, disconnected from the rest.
You might able to deduce what's going on without taking it all apart though.
If you connect a jack cable to the cab and touch a 9v battery between the sleeve and tip, the working speaker(s) should move forward. If you bung up the port, the non-working speaker(s) should move backwards slightly, by a lesser amount as they will be sucked back a bit by the reduced pressure caused by the good one going forwards.
Caveat: I've not tried this, but I think it should work!
Let us know how you get on.
Cheers
MBA