You can measure the impedance of a driver with a wheatstone bridge, a power amp, sig gen and ac vtvm at any frequency you wanted if you wanted to be really fussy.
You'd do this if you were designing a passive crossover and wanted to know the exact impedance of a driver at the xover freq, or if you were designing an impedance equalizer as part of the xover.
You can get devices made for the job now, and no doubt there have been articles in electronics mags for doing this.
Sometimes if you had a xover that had different orders for bass and tweeter you would deliberatly reverse the polarity of one of the drivers due to the phase differences at the xover freq, so if you see this it might not be a mistake.
Why do you want to know the impedance, if just for a quick check then Nige's idea works well, an ohm less than specified.
A check CD is good for testing phase, they often have a soundtrack of a bass freq in and out of phase, it should sound louder during the inphase track.