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...