You can view a web site's HTTP headers using a Firefox add-on called "Live HTTP Headers."
Here's the page for the add-on: http://forums.xisto.com/no_longer_exists/
Once the add-on is installed, you'll be able to view the server header information by right-clicking on the page and selecting "View Page Info." In the "Page Info" window, you'll see a new "Headers" section.
This is what it looks like on my laptop: