I was reading a post on a French BMW forum (bmist.forumpro.fr) and found a good link to a page detailing how to test the Hall sensors.
This is an English page. The K bikes are using the same sensors as the R bikes (HONEYWELL S&C - 2AV54)
Here it is: http://www.ebbo.org/2av54.php

Bert

Good one Bert,
Excellent write-up and I note Honeywell are listing the 2AV 54 as being replaced with the 2AV54-J but their website is crap.
Regards,
K-BIKE

I just checked the wiring of the Hall sensors on the K bike. It seems that the color of the wires is different on a R bike. I think the K bike has also a round plug instead of the flat one on the R model.
Here is what I believe is the wiring on the K100 (I never physically checked it but just followed the wiring diagram). The test explained on Ebbo.org still apply to the K100.

Good on you for spotting that!
Regards,
K-BIKE

I think the plug is indeed round, but also has 5 pins. I'll have a look for a picture.