How do I measure impedance of my guitar pickups?
Since the pickups have both a reactive and resistive component, it's not just as simple as measuring resistance alone. The (assuming standard coil) pickups are inductive, will have some resistance and some stray capacitance also.

The impedance is calculated using both components, using 

$$Z = \sqrt{R^2 + X^2}$$

The resistive part is easy, just measure with a multimeter on ohms. For the reactive part you need an LC meter (which your multimeter may have, but most only have the C testing capability).

Depending on the LC meter, it may do all measurements for you and display reactive, resistive and total impedance, or it may just display reactive. You will be interested in the results over the entire frequency range (for a guitar pickup say 100Hz to 5kHz).

Or you can use a scope and known impedance/signal to drive the coil with and do the necessary calculations.  

IIRC, wound guitar pickups are usually in the 5k - 100k range at audio frequencies, but my memory is fuzzy so this may be a bit out.