How to measure HTTP cache hit rates?

Is it possible to detect HTTP cache hits in order to calculate a cache hit rate?

I'd like to add a snippet of code (JavaScript) to a HTML page that reports (AJAX) whether a resource was available from a client's local cache or fetched from server. I'd then compile some stats to give some insight on the effects of my cache tuning. I'm particularly interested in hit rates for the very first page of a user's visit.

I though about using access logs but that seems imprecise (bots) and cumbersome. Additionally, it wouldn't work with resources from different servers (especially Google'开发者_开发问答s AJAX Libraries API, e.g. jquery.min.js).

Any non-JavaScript solution would be well appreciated too though.

There might be some easier way, but you could build a test where javascript loads the element and you record the time. Then when the onload event fires compare the times. You would have to test to see what the exact difference between loading from cache and loading from the server is. Or for a whole lot of items have the javascript load first record the time. Then record the onload events of everything else as it loads onto the page. This may not be as accurate though.