tracking payload size of third party content

[calumfodder]

Is there a way to track the size of payload that 3rd parties such as adservers are adding to your site, or rather the end users perception of your site as it is delivered straight to the browser (in most cases), via some nifty javascript?
Utilising a tool such as webpagetest.org to monitor/test a site requires an element of luck to catch an offending ad or intermittent large file.