Search ebay/etc for "EC/TDS meter", it's ~10 USD. Cheap ones do work, no worry. Keep your drain water at a stable EC, somewhere between 500 and 2500 uS/cm, and you're golden. If the drain water's EC is low, use irrigation water somewhat richer (+500..1000 uS/cm). Some tap water is already high on minerals, in that case use mainly nitrogen-rich solution.
Btw if your plants look healthy, no action is required.
Btw if your plants look healthy, no action is required.