While there are test kits out there for the various elements that plants require, sometimes the cost of the test kit is not justifiable.
For example, we know that plants require CO2, and there are CO2 sensors available, but they cost in the thousands of dollars. That kind of price is not justifiable (to me, anyway).
To answer your question about dosing, Tom Barr designed the EI (Estimative Index) method of dosing to provide an abundance of nutrients to plants so that they are never deficient. A 50% water change on a weekly basis resets these nutrients to a lower level such that a dangerous level is not accumulated.
What are the effects of "dangerous levels" of nutrients?