Set your multimeter to the 20 Ohm scale, and place one probe on the wire to the tag of the pot (green?) and the other to the earth wire (black). This should give you the reading you're after, assuming that your pickup is wired liked using 2 conductors.
To check the output ohm's, try putting the probes on the output jack, remembering that one wire goes to the tip, the other wire to the jacket of the jack plug.
I think... :?