Did it come with a in-line TDS meter? If not I would suggest you get a couple. They come with two probes. The first set of probes set between the incoming source water and after your last filter before RO membrane. The second set of probes set right after the membrane and right after the DI canister. This way you'll know when to change out filters and RO membrane. The DI resin can be color changing but having the TDS probe after te DI resin canister can tell when that needs to be changed too, I change mine when the meter reads 001.
The link will help you understand if the above is confusing (still working on my first cup of coffee).
Dual Inline TDS Meter DM-1 HM Digital
Dave-