The usual test for salt is a titration test using a chromate indicator and silver nitrate as the titrant. The test is slow and easy to do wrong, however. My recommendation is to use the AquaChek salt test strips which are actually reverse titrator strips that use the same silver nitrate chemistry but without the chromate indicator. The strips are extremely accurate, easy to do, and inexpensive. The only drawback is that they are slow (5-10 minutes) but if you are doing a battery of tests then test salt first and check it last.