I was convinced that liquid fertilizer labeling use the % weight by volume convention. However now I bought a magnesium-sulfur based additive. The label clearly uses % weight by weight. Weight by volume is indicated in brackets (g / liter) and by multiplying the density with the % weight by weight the value in the brackets is calculated.
In this case it is clear that % weight by weight is used. However how can one know if there is only % given without any further information?