I use a Firefox addon called "TamperData". It's really good for stuff like this. I just go to a web page, open TamperData, hit <ctrl>refresh and it will give me

I use a Firefox addon called "TamperData". It's really good for stuff like this. I just go to a web page, open TamperData, hit <ctrl>refresh and it will give me a list of all the requests that are made. I can then go and take a look at the full set of HTTP Request and Response headers for each individual request. It lets you intercept and modify request headers on the fly too. Very useful tool.