The best way is to follow the filter change schedule. The sediment filters fill up with sediment and restrict the flow through the filter so they are usually easy to spot when they need changing.
There are TDS meters for testing the membrane to see when that needs to be replaced.
Liquatec TDS Meter PM-3000 to Monitor Total Dissolved Solids with a Handheld TDS Meter Reverse Osmosis Inline TDS Meter to Monitor Total Dissolved Solids in RO System Water.
Thank you,
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