I just recommend picking them up on occasion and looking them over.
Check for:
Eyes clear & alert, comb is not off color, no nasal discharge, sneezing, panting. Feathers should be nice & shiny, no mites or poultry lice on skin, they shouldn't feel bony, scales on feet should be smooth and underside of feet should not be discolored. They should eat & drink normally & poop should be solid brownish black lumps with a white crest most of the time. Make sure vent is pink & relatively clean, and no feathers missing around the area.