Experience lol.or you can test your run off with a tds meter. If the run off is more than 300 ppm higher than the solution being put in than you have salt build up. I flush with solution when i flip and 4 weeks in just to be safe. I also flush with solution if they stop praying. Turger pressure is the preassure created by plant osmosis. Water wants to travel from areas of low salt to areas of high salt and a drop in pressure can mean salts are building up.