Chickens are 'very' good at hiding their pain unless you know what to look for.
How many birds do you have and generally how old are they?
If they are hand-tame, I would recommend regular handling at least once a week. Pick them up, do a quick check for mites on the back and neck. Check their musculature on the keel/chest. Check their feet for signs of bumblefoot. Make sure their feathers are tidy, no clumpy poo around the vent and observe their activity level. Check their crop. First thing in the morning it should be empty; full in the evening and typically always have at least some food in there during the day.
Some hens are quite mellow where others can't stand to sit still. Any deviations from the norm should be suspect. Any hens off by themselves should be suspect too. Weird eggs in the best box? Best to find out who they're coming from and keep an eye out.
You can learn plenty just sitting with your flock and watching them.
A hen in pain may be fluffed up, may put all their weight on one foot, may stop eating, may look like they're 'going through the motions' when flocking or less enthusiastic about treats. Knowing their personality and picking up subtle changes can give you a bit of a heads up, but they can go down hill very fast and die suddenly too.
If you had no idea it was ill and are worried it could be something contagious, having an autopsy is the best way to protect the rest of your flock.
it sucks . They really just do die suddenly sometimes and it's no one's fault. Sorry
