Precision Nutrient Inspection In a Feed MIx For Hydro

Is there a way to check the % of what's in your water reservoir? I am assuming plants pull in more of one thing than others depending on where they are in the grow and you feed them but other than visually checking your plants and making educated guesses on a strain most of us only grow once or twice. I check the sap PH and that tells me what the plant is lacking but it doesnt really tell me how much its lacking or if its off balance. I do over think things frequently though