Yes, potassium is usually just estimated. However, I have offered to do potassium and iron testing for users on this forum with a specialized instrument that gives very accurate results. The post is in the swap and shop section (search function: potassium and iron). The commercial potassium test kits out there are turbidimetric and again, difficult to judge by eye at the levels needed in a planted tank.
Sulfates are not really a problem - they are at such low levels (<100ppm) normally. In saltwater they are much higher (>1000ppm) and don't cause problems there.
72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias
2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.