These are some of the common health problems that chickens have, some my chickens have experienced and some they have not. Hopefully you spend some time on a daily basis with your flock, otherwise some conditions might not be caught as early as possible.
Inspection includes looking at their feet, legs, vents, feathers, etc.
Watch how they interact with each other, and how they walk. When you pick up your chickens, look at the bottoms of their feet, and fan through their feathers, so you can see down to the skin. Inspect the vent area, too. Worming If your chickens free range, they will need to be wormed at some point. My first six chickens were wormed for the first time after 2 years.