How to measure load detection for ballast output 18W
If the current is AC, you can use a current transformer.

These comes in different sensitivity to measure different range of current range like the CR4110-0.5 which has a rating of 500mA which should be good enough to measure down to the mA range. 

If that's not enough, you can make the wire pass several time in it to increase the sensitivity.

EDIT:

Just saw the 44kHz frequency range, that is beyond what most sensor will be able to handle, but you can just place it before the ballast on the 220AC lines, it will work the same, you just need to check what is the delta V when tubes / no tubes.