Focus on standards of performance. We never can reach absolute zero with microbial concentrations. The performance of the treatment plant is best evaluated by comparing the source water with the filtered water. However, for cleaner source waters with turbidities less than about 5 NTU, turbidity reduction becomes an increasingly inadequate predictor of overall particle reduction. Football games are won by blocking and tackling, proper execution of fundamentals.
Safe water is produced by proper execution of fundamental treatment processes.
Now is not the time for complacency, now is the time for increased vigilance. Reliability and consistency of treatment is the key. This means that sufficient resources need to be allocated for operation, maintenance and capital improvements. See attachment - Assessing Treatment Plant Performance. The simple answer is by observing them through a microscope.