You could use ion selective electrodes (very expensive). You could use a colorimeter, but you'd need to understand the chemistries involved for each thing you're looking for which could take a long time, and requires chemists to setup the tests.
Personally, I am not using any of this. When making DIY nutes, I know pretty well how much of each chemical goes into each stock solutions by the scales measurements. (ion selective electrodes would be fun to play with, but I just have an EC meter)