It depends on how great a detail you want to get into, but lab quality testing equipment is available, or you can send off plant tissue samples for analysis.
For most hobbyists, though that sort of attention to detail is going way overboard.
You can present the info as a chart or graph with the level of a particular element on the left, and plant growth across the bottom.
At the far left is zero dosing. No detectible level of the thing you are testing.
Plants will die without that element.
Moving slightly to the right there are low levels of something. Barely enough to help the plants out under low tech situation. This low level of the element in question leads to deficiency in a high tech tank when the plants are more demanding. The plants may grow slowly.
In the middle of the chart is a very wide band of 'acceptable levels'.
Plants will grow just fine. There is enough of the element present to supply all its needs, not so much that it will inhibit the uptake of something else, or be toxic to the plant or livestock.
It is this band that Tom aimed for when he worked out the EI method. Dose all week and gradually get more and more nutrients in the tank, but do not get too much. Then a water changes takes it back to the right, almost at the edge of deficiency. The line representing plant growth is level throughout this range. As long as the minimum needs are met there is not more growth with more fertilizer.
Farther right than that is excess. The element may be toxic, or it may competitively inhibit the uptake of some other element. The line representing plant growth is usually growing down at this end of the graph. If some one element is present in near toxic levels there may be ways to deal with it, for example growing the plants faster, then trimming. The trimming removes that element from the system. Better not to get into that situation by dosing anywhere in the safe range, not at this high level.