Once the RO membrane system is placed back online after CIP, you can get a relatively good idea by comparing post-cleaning feed pressures and differential pressures for each stage to pre-cleaning values. The most reliable method would be to observe the trend in your normalized permeate flow chart, since that corrects for changes in temperature, conductivity and % recovery.
See also:
What is the procedure for cleaning RO membranes? What CIP chemicals should be used?
What is the relationship between pH and alkalinity? How are they different? How does temperature affect them?
Our RO plant doesn’t have a neutralization tank for our spent CIP solution. Can we just neutralize while circulating through the membrane system?
Under which conditions can antiscalants cause RO membrane fouling?