How to know if the electrolytes are balanced.. I would go for symptoms, wellbeing-energy (or the lack of it); then do as V, me and others; try, test, fail, try again. I would look at my diet / supplements and roughly estimate what deficiency is most likely.
Like cramps for instance. Typically we think magnesium; which is good. But then you come to a level were almost all cramps are gone, but not all of them. Then it is time to up the potassium, or maybe reduce calcium intake (as it promotes contraction)Took me a couple of years to find out of magnesium.
I have done many mistakes, and overdone a few things; this taught me most of all.