You need a PPM meter. Check your Res. every other day and see how much and what your plants are sucking up.
Mix up your normal nutrient feeding and check its PPM. (lets say its 850 PPM)
Come back after a day and check the PPM again. It will have changed. If your plants are sucking up more nutrients than water your PPM show it. If your PPM has lowered plants are taking in the nutes if the PPM risen then the plants r taking up all water.
Hope it helps
MJ