Any way to know when content has been updated

Is there any API to call that can return the date/time the last time content was updated? For performance reasons we’d like to cache the content at the app server level unless something has been added/updated.

You can use E-Cache mechanism.

Every API endpoint returns the Etag header which is for a content item the version of the content. if you add the If-None-Match to the request with the last etag/version you have retrieved you will get back an 304 without any payload.