To test if the RODI unit is working, I purchased a TDS (Total Dissolved Solids) meter. It is a simple device, just hold the probes in the water and wait a second. Results are fast, and appear to be very accurate. I did a couple experiments to get a better understanding of TDS in different environments.
In my existing 40 gallon breeder, the TDS reading was 385. Not really surprising. In my house water, which is filtered, the reading was 250. Also not terribly surprising, considering our city water is liquid concrete. The reading in some water I had made from the RODI unit was 7.
Overall, I can see how useful the TDS meter will be moving forward. It is just another tool to help us keep track of what is going on in our tanks.