I would first question what the TDS is of my starting media.
Then, how much you are feeding and how much your plants are using or are able to use what you are feeding them. If they are not up taking what you are giving them then salts will accumulate in the soil and if you are not flushing them out, they will remain. When I started this years grow I conducted a slurry test of my soil and water. What I found was the depleted miracle grow and water was at 220 ppm. my nutrient mix runs 480 ppm. I am not flushing my grow as I have no drain holes in my grow cups. So everything I put in either gets used or stays in the media. I will do another slurry when I plant out and see how well my nutrients were utilized by the plants. I started out fertilizing pretty heavily due to a misunderstanding of the feeding schedule and was feeding every 5-7 days. That was unintentional test #1 for NOTG nutrient line. NO FERT BURN! from overuse.
Â
As far as being able to tell other than an EC meter. You can wait for indications from your plants but, I think when your plants start to tell you it is almost too late or you will lose valuable growing time from then on getting control of the situation.