You can buy a hardness test kit at Lowes, HD or Sears, they are very inexpensive and help you set your softener so it is doing it best job without wasting salt or allowing hard water to pass through.

As long as you are at 0 TDS or very close to it your system is functioning as it should. Prefilter andcarbon block changes are normally at 6 month intervals in most cases. If you have sediment. iron or particulate problems though I would go by pressure drop. I use two presure gauges on my personal RO/DI, the one that came on the membrane housing which is post prefilter and carbon and an additional one I installed before the prefilter so I can monitor pressure drop at a glance. If I start to see more than a couple psi difference between the two gauges its time to replace the filters. With the single gauge on the membrane housing I have no idea what my incoming pressure is so two gauges are good insurance.

I am not a fan or doubling up on filters such as two prefilters or two carbons, each thing you add in front of the membrane has an associated pressure drop which reduces the membrane efficiency. I would rather invest in better filters rather than more filters.

I use a 0.2 micron pleated prefilter and a single 0.5 micron 20,000 gallon Chlorine Guzzler type carbon block and after 23 months was still seeing only a 2 psi drop between incoming and post filters. Again thats with a tap water TDS between 650 and 800 but a low sediment or particulate loading.

Membranes normally go 18 months to 3 years but some people report over 10 years with Spectrapure systems. Randy Holmes Farley reported 10 years on a single membrane in a thread on RC a few years ago so it can be done. One reason this can be done is they use absolute rated prefilters in low 0.2 or 0.5 micron sizes versus others 1, 5 or even 10 micron nominal rated filters which are not as efficient. I believe www.buckeyefieldsupply.com also carries the pleated prefilters and 0.5 micron carbon blocks too. If its never reaches the membrane it does not have to be removed by it so the membrane lasts longer and functions better.