Comment by TuringNYC

You can kinda tell based on their actions (sometimes). We haven't really needed to bring them to the vet for illness, but once for amputation of an infected + hurt toe. Additionally, if they get parasites (typically mites), they need anti-parasitics. My wife has done a ton of research into identifying chicken issues so she is always on the lookout.

We've had other times where one might appear a bit sluggish, but then the next day are back to normal. Probably ate something bad?

There are several conditions that have visible consequences, such as injuries, malformations, anomalies or a general affectation of their appearance. Plenty of those can be quite disturbing for someone with no experience.

Storey's Guides are a good starting point: tohttps://www.barnesandnoble.com/w/storeys-guide-to-raising-ch...