If you want to find out how much salt is in it you'll need a ppm/tds meter. pH your water to 6.0, measure the ppm of the water, soak your coir in the pH6 water and then get rid of the excess water into a bucket or something similar. Measure the ppm of the water again. If it doesn't go up much then there's not much salt, if it goes up a fair bit then keep rinsing with pH6 water until the ppm of waste water is almost the same as the pH6 water before rinsing the coir.