I've been looking at my headers the old-fashioned way with curl -I and wget --save-headers, but I notice Chrome and IE have some pretty advanced site diagnostics built-in now. Here I am using Chrome's F12 to check my HTTP headers which I just changed.

Downloads:
/videos/view-http-headers-with-chrome.mp4
/videos/view-http-headers-with-chrome.webm
/videos/view-http-headers-with-chrome.ogv

Whoops! I messed up those headers! I fixed them after I made the video, and then I used Chrome's F12 to check them, and this time I got them right.