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.