This is what ETags are for. Upon a user's first visit the server should return an ETag uniquely representing the current version of the page. The browser will cache both the page and the tag. Upon subsequent page visits the browser will send an If-None-Match header containing the tag for the version of the page it has cached. The server should compare the incoming tag with the tag for the current version and return a "304 Not Modified" response if the tags match or a full response with the newer tag in the ETag header if they don't.
a drawback of relying on ETag is that if a page is visited frequently, then the cache validation "If-None-Match" request still being sent and takes bw+latency+computation+etc and I suspect that if the connection is broken or status 503/504 is responded, then neither the cached page is shown.
my understanding is that he whant to refresh the page only if it's known to be changed and always use the cached version otherwise.
