Hi fi speaker impedance
The impedance of a loudspeaker depends on frequency, crossover network, how many drivers it has, etc.

If you measure it with an ohmmeter, you'll get value which will be the DC resistance of the woofer, plus any coils in the crossover lowpass filter which are in series with the woofer. 

A 8 ohms woover will usually have 6-7 ohms DC resistance, add 1 ohm for the crossover coil and wiring, so you'll probably get 7-8 ohms on your multimeter. Maybe 6 ohms if the woofer is unfiltered, or if this is a one-driver speaker without filter.

If you get half that, it's a 4 ohms speaker.

That's a bit rule-of-thumb-y but it'll work in most cases.