A reverse osmosis system rarely fails all at once. The membrane loses its ability to reject dissolved solids a little at a time, and the water keeps coming out of the faucet looking normal. By the time the taste changes, the membrane may have been underperforming for a long stretch. The useful habit is to watch a few simple measurements so you notice the slide early and can ask the right questions before you approve any work.
Signs to watch in the tank and the drain
Start with how the system behaves, since this takes no tools. Note how long the storage tank takes to refill after you draw a few gallons, and how often the drain valve runs. A membrane that is slipping usually makes less product water per cycle, so the tank refills more slowly and the system may cycle more often to keep up. Write these observations down on a weekly routine so you have a baseline to compare against. A single slow refill means little. A steady trend over several weeks means something.
Measure the feed and the product water
A pen-style TDS meter, the handheld kind sold for water testing, gives the most direct early warning. Test the cold water entering the RO unit, then test the water from the RO faucet. Run the RO faucet briefly before sampling so you measure current output and not stale water sitting in the line. Take both readings at the same time, with the same meter and the same method, so any difference comes from the system and not from the test. A healthy membrane removes most dissolved solids, so the product reading should sit far below the feed reading. If the product reading creeps upward over time toward the feed reading, the membrane is passing more than it should. Ask your technician what product reading they consider acceptable for your particular feed water, because the right number depends on your source.
Rule out the cheaper causes first
Several problems copy the look of a failing membrane. A clogged prefilter restricts flow and slows the tank refill while leaving the reject behavior largely unchanged. Low feed pressure does the same. A storage tank whose air charge has dropped will make the faucet feel weak and change the fill pattern. A closed valve or a kinked line can also cut output. Your feed water may have changed too. If the incoming dissolved solids rose, the product reading will rise with it even when the membrane is healthy, so track the feed reading alongside the product reading. Check the prefilter condition, the feed pressure at the system, the tank air charge, and the feed water itself before you assume the membrane is done. Ask the technician which of these they checked and in what order.
What to ask before you approve work
Before anyone replaces the membrane or the whole unit, ask for the readings they took and write them down with the date and location of each test. Ask whether they compared the feed and product readings, whether they checked the prefilters and the tank pressure, and whether the replacement covers the membrane alone or a full set of cartridges. Ask what the warranty covers after the change, and whether the work includes a before and after reading so you can confirm the unit is performing. Ask how often they recommend checking the readings going forward and whether you can repeat the check yourself with the meter.
Keep a simple log
A notebook or a note on your phone is enough. Each entry should include the tank refill time, the drain pattern, the feed reading, the product reading, and anything you notice about taste or flow. Monthly entries kept over a year will show a trend that a single check cannot. When the numbers drift in the same direction across several entries, schedule a service visit rather than waiting for the water to taste wrong. The goal is not to diagnose the system yourself but to arrive at the service call with evidence. If you are still choosing a provider, the Best Water Companies STL award winners list is a starting point for finding local companies to ask.