You can't just measure the DC resistance, that's not relevant at all.
The easiest way to measure some impedance is to send a standard 50 / 60 Hz sine signal to the speaker (e.g. produced by dangling an unshielded wire from the amplifier input1 and turning up the volume), and then measure both AC voltage parallel to the speaker and AC current in series, which at mains frequency any multimeter should be able to do. The impedance (more precisely, its magnitude) at that frequency is the ratio U / I.
However, using only a single frequency isn't really reliable. You might well obtain a much higher result than you should, since the nominal impendace is basically just the minimum of the frequency-dependant impedance. To do it properly, you therefore need a function generator (any PC with suitable software will do) and a multimeter that allows for AC measurements over the entire audible range. The measurement itself remains the same: supply a signal (varying frequency sine) from the amp, and measure voltage in parallel and current in series.
See the Wikipedia article for more information.
You might need a preamp for this. Needless to say, an improvised mains antenna offers horrible
signal quality, you might actually get more of some higher harmonic than the fundamental frequency.