Behavior, more than any other thing, is the best gauge of how chickens are feeling. If your chickens are behaving as they normally do, they are probably fine.
What have they been eating? Some things will color the poop sometimes. Feed them purple cabbage, for example, and you'll see some spectacular poop. Smelly, too.
Green poop can sometimes signify the presence of bile. But that usually only occurs when a chicken is quite ill and off their feed. The color of poop alone does not necessarily indicate illness.
The thing to look for if you suspect a chicken is sick is one that is moping around, standing in one spot with tail held low and flat, usually facing a wall or corner. They do not want to eat or interact with the flock. Eyes may be at half mast or closed. Sick chickens don't talk. When a normally chatty chicken goes mute, that's a sign of illness. If they have watery green poop in addition to all this, then you know your chicken is sick.