www.youtube.com/watch?v=rLXlIK1MJ9s
Here is a really short video about testing water.
For hardness, I would send it for testing or get a real test kit, specifically for DKH, you can get one at an aquarium store. I don't trust my TDS meter for hardness, but maybe it's, ok?
Soft water will save you a lot of maintenance troubles and chemical costs. It will also extend the life of your RO membranes, especially if you have chlorinated municipal water.
Also note at the end of the video, he tells you anything over 7 grains will plug a tankless water heater pretty quickly.