pro-actively check for specific updated content

Cache busting is using 'version tag' query parameters to differentiate versions of static content (js/css/images). Whatever is generating the pages needs to do this. Sometimes the version gets created every time a new package is released, sometimes when the webserver restarts (we do it both ways).

You end up with this kind of thing when the page loads;-

https://test.com/theme/js/ajaxportlet.js?t=1385450658000
https://test.com/theme/css/stuff.css?t=1385450658000

This way you can have far-future proxy/client Cache-Control headers on your static content.

I'd google it and talk to your app guys about whether they feel like doing it.