Gray water: It is water that comes from the kitchen sinks, bathrooms, bathing water, water from washing clothes, utensils, or any other wastewater which does not contain feces of humans or other animals.
Blackwater: It is now apparent that it is water which contains feces of animals and humans, it is water from urinals, toilet seats, commodes, etc. is called black water
And to tell the difference in the sewer is hard because it generally mixes in the sewer, if not provided with different outlets.
I hope this helps!