[John N.]

I've had black spots on my narrow leaf java fern too. Changing nothing else but increasing the nitrate and potassium dosage slightly, the new growth sprouted without any more black spots. Some of the black areas are also result from older leaves simply dying off and giving rise to new leaves.

I wouldn't recommend looking for a test kit to measure those elements. As mentioned previously our hobbyist tests kits aren't very effective or accurate for what we want to do with them. Instead see how a little increase of fertilzers affects your plants.

-John N.