How do you tell a healthy hen???

I would say they should be appropriately heavy for their size, nicely feathered (no bare spots or poopy butts) and generally look healthy. Check their eyes for crawly things and no cloudiness, discharge, etc. Pick one up and blow the feathers back so you can see the skin, to check for lice or other issues. Poops should be, well, chicken poops... not runny or off color, etc.

 

Check their legs and make sure they are nice and smooth scaly. Also, the healthier a hen, the thicker thier egg shell will be.

 

Lordy, then my chickens must be in the pink of health! There are days I have to whang the eggs against the granite counter top to get them to crack. My friend has taken to calling them "hippy eggs" after overhearing her omlet loving husband cussing up a storm one morning at "these g-d d-d hippy eggs" that wouldn't break!

(He loves'em though, and hasn't once turned them down.)