soft water tester

The RV Filter guy told me that the TDS units will not measure hardness - and he sells them.
See HERE

He is usually pretty accurate although his recommendation for a magnetic water softener based on a one season very non-rigorous test is silly.

If you want to test the performance of your softener, you want a hardness test kit. The water would be considered soft if the hardness is less than 1 grain per gallon.

There are no practical digital test kits for hardness. Typical hardness test kits use a drop-count titration method. You add a color indicator to the sample; if the color is red the water contains hardness. You add a second chemical (EDTA) to the water dropwise, with each drop typically representing 1 gpg, until the solution turns blue.

Try Amazon.

Although not digital, I bought a very simple test kit at my pool supply store. Put a little water in a vile, 2 drops of a solution in it and a few drops of a second solution to get the hardness. Very accurate and simple to use.

Google or Amazon. We're so helpful.

just Google for it and you will get many