Situation: the new server for the site is ready. How can I be sure the sizing is being made correctly and the machine is able to improve the performance of the previous server?
With a simple (and stupid) trick!
Add a hidden iframe to the live site that makes the same request to your development system. (ex: www.site.com/some/url/123 -> dev.site.com/some/url/123/).
Place the code in a common section, that is repeated in all site pages (es. footer).