The simplest way is from the supplier of the DI water columns themselves, many of them will have at the last tank on the output will be a simple what we call “go no-go” red and green light, those are set typically for either 2 megaohm or 1 megaohm. You can even get some with lower rating too down to the k-ohm if need be. If there are other requirements, there are portable instruments available from companies like Myron L that has monitoring systems.