If you watch your chickens closely, after a while you will be able to learn their body language and know when they are happy, sad, broody, ill, scared, mad or something different entirely.
Chickens that are happy will be active and scratching around in the grass, laying in the sun--yes, they do that, too!--or taking a bath in the dirt. We don't recommend YOU try that chicken behavior--for humans, water baths tend to work a little better.
Chickens, however, use dust baths to help keep themselves free to parasites. They also may be looking for a good, private place to deposit their eggs. Happy chickens who go broody will try to sit on their eggs all day to hatch them, and will protest if you try to encourage them to leave.
If your chickens are ill, they will not be very active, and will often not leave the coop to do their usual things. Their feathers will be ruffled or puffed out, and their combs may be pale.
Chickens that are scared will try to hide or run away.
Some people think chickens don't get sad, but they do, especially if they don't have enough room to perform normal chicken behaviors like scratching and so forth, because their instincts are frustrated. This renders them helpless to do the things they need to! When they are sad, often let out very sad sounds that are unmistakable, the same way your dog might whimper if he is unhappy because he is hungry or needs to go outside.