Check the scale you're using on your multi-meter. You want to be reading meg ohms (1000K ohms). If you're using the wrong scale or if the meter isn't that sensitive then it will just read infinite resistance.

You can't completely decide if a knock sensor is good by checking that the resistance is within the normal range. To be certain, you'd need an oscilloscope to check the waveform generated under specific knock conditions. Part swappping without changing any other engine running conditions is simpler.

Do check that the bolt is properly torqued to a light 10-12 ft-lbs (use a little LocTite blue). Do clean up the electrical contacts and pack the connector and boot with dielectric grease to keep out moisture.

FYI From the brickboard RWD archives [posted by John Sargent, Oct 31, 2000]:
> According to abe [Volvo master machanic and guru], the later knock
> sensors have a resistance ranging from 2 to 8 meg ohm. The earlier
> knock sensors are open circuit. He feels the resistance test is not
> reliable, and says the knock sensors rarely fail, except when
> overheated.

For even more information check out this article in the Testing a Knock Sensor in the 700-900 FAQ area.