You can check with your RO system supplier as to what level of hardness they consider acceptable. Most softeners are more than capable of reducing hardness to 100ppm (o.6 grains per gallon) or less, if they are well maintained. If you are seeing levels which exceed that, you can have your maintenance folks or the softener service person check the length and frequency of your softener cycle compared to your water use to see if adjustments are needed.

"Dip and Read" water hardness test strips, sensitive to 10ppm or less, are now available commercially through dialysis dealers. Of course the liquid water hardness tests are readily available from any water supplies or equipment company. The liquid tests have sensitivity down to 1 or 2 ppm, if you don't mind doing a little extra work to get better sensitivity. Testing should be done at least once per day, usually in the morning at the same time as your Total Chlorine tests. Take the sample from any convenient location just after the water softener. Some units test before each shift. (although once per is really adequate for most units).

And the main reason for testing IS to prevent premature clogging of the expensive RO membranes. It is cheaper to maintain your softener than to replace RO membranes!