Yes sulfates can be measured. Not usually by the typical hobbyist though. If a barium chloride solution is added to a test solution, the sulfate precipitates as BaSO4 - this is measured turbidimetrically (light scattering). Just like potassium measurements, it is not very useful at low concentrations (as in freshwater) and is hard to judge by eye alone.
I've not heard a reason to worry about sulfates at the levels typical to aquariums. The concern with KNO3/K2SO4 is you want to keep the K to NO3 level right.
I meant more with respect to dosing - if you use KNO3 to up your nitrates and no other source of potassium, your potassium will be low. You then need to add K2SO4 to make up the difference. You want 10ppm NO3 and I think >20ppm K. KNO3 alone will not produce this ratio.
So is K2SO4 a safe way to top up the difference and fix the ratio?I don,t know much about sulfur,won't the excess sulfates harm?
In this case you will have to measure your pottasium,I don't know any commercial kits for this so I suppose you have to calculate by the molarity.Correct me if I am wrong
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.