How can I detect and handle stale cache responses in an Express application using Cache-Control validation?

When building an Express API, I need to ensure that clients can detect when a cached representation has become stale and trigger revalidation without downloading the full payload.

The goal is to configure responses with appropriate Cache-Control directives so that shared caches mark entries as stale after a defined period, yet still allow the client to send conditional requests (If-None-Match or If-Modified-Since) to verify freshness.

I am uncertain about which combination of directives (max-age, must-revalidate, no-cache) reliably causes a stale state while preserving the ability to validate, and how to observe the resulting 304 Not Modified or 200 OK in logs.

How can I set up Express middleware to emit the correct headers? How can I verify that a stale cache entry leads to a validation request? What logging or inspection techniques expose whether a response was served fresh or stale?