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?