If those who are sent to draw water begin by drinking themselves, the army is suffering from thirst.
Fetching water away from the safety of others has some hazard and so soldiers on this duty are unlikely to pause to drink unless there is a lack of water back in camp and all the water they take will be quickly drunk by others. When this is true, then there is likely a lack of water in the main camp.
This is a simple sign that is easily missed. By watching and thinking why people act as they do, you can surmise much.