There's no way to check the watermaker at the dock. The amount of salt makes no difference to the chemistry, just the pressures and output levels. The watermaker membrane sizes are smaller than almost all viruses, and bacteria have no chance of making it through. Any contamination found will be in the tanks. What kills membranes are chlorine and diesel. Water around the docks have both diesel and lots of sediments that clog up the 2 pre-filters. Running the watermaker while underway will ensure that it keeps running.
I've been using watermakers for over 15 years for every drop of my water aboard and if you treat them well they will just keep on running and producing clean water.

[Linked Image]