There are many ways to check, but the most effective ways are human observation and intuition,
There should not be too much smell from the aeration tank
Treated water before chlorination should not smell
Treated water when kept in a container for more than 2 days should not change colour
Treated water when kept in a transparent bottle for more than 2 days should not form algae
Apart from this there are other ways to check the performance of the plant based on activated sludge characteristics,
The aeration chamber water with the bacteria/ activated sludge should be in brown color (light- dark brown)
Activated sludge should not be grey, black or yellow in colour
Water collected from the collection tank in a 1 ltr bottle should be brown in colour and when kept for 30 minutes
should form clear water on top leaving the brown particles (activated sludge) to form flocks and settles to the bottom of the bottle.
There should be a grassy smell in the water collected from the aeration tank