How do you Know if you Have a Salt Build Up in your Soil

so my plants are showing the signs of being N and Mg deficient so im guessing lockout and im guessing I have a salt build up so when I water next (ph 6.4 always) I will test the run off but is there something specific I should be looking for as far as ph or ppm of the run off? either way I will flush 2 gallons of water per gallon of soil and feed at 1/4 strength at the end...peace and thanks guys

Look for yellowing leaves with brown spatchs and or dots. I would go ahead with the flush.