on that unit you should be able to get 2 readings of TDS. one reading should say "in" this would be before any filters and will tell you what kind of water you are working with. the second reading should say "out" this will be after all filters and if the unit is working properly should be 0. to find out if the RO membrane is working properly, remove the DI cartridge and replace the cover to it (empty/ without the DI) run the unit for a few min. then take a TDS reading of the water going in, then a reading of the water going out. let just say that the water going in is at 100-TDS and out is 10-TDS that would mean that the RO membrane has a 90% rejection rate.if your rejection rate is 85% or lower i would say the membrane is shot. most RO membranes over 75gpd that are working properly have a rejection rate of about 90%. most membranes under 75gpd have a rejection rate of about 98%. thats why most people will recommend 75gpd membranes because the membrane does most of the work and the DI only has to clean up the remaining 2%= longer DI life. with a 90% rejection the DI has to remove the remaining 10%, let say your source water is 500-TDS, DI would have to remove 50-TDS=using lots of DI | 
