Originally Posted by chimchim
No, you can't.
Impediance cannot be measured with a DC source like a multimeter.
Easiest way (like posted above) is to open up the thing and hopefully it's listed on the driver magnet.
BTW, 4 or more ohm impediance will work on all (4 ohm) car stereo equipment. 8 ohm just won't sound as loud.
Actually, you can measure it using a multimeter. Impediance is resistance varying with frequency... therefore, the resistance is never the same, but it will be pretty close to the actual number. For example, if you get a reading of 3.7 ohms, then you have a 4 ohm speaker.
And I wouldn't go as far to say that 4 ohm will work on all stereo equipment. 8 ohm speakers are sometimes extremely useful and much more beneficial than 4 ohm speakers. Many times you can wire 8 ohm speakers to sound just as loud as 4 ohm speakers.