ergophobe - 5:58 pm on Mar 15, 2011 (gmt 0)
You want to check this by checking headers.
Unfortunately, some anti-virus software (Symantec, Norton for sure, at least on my computers) will strip the browser request header saying that the browser will accept gzip. This is because these AV companies believe that it will bog down your CPU uncompressing everything before doing on-the-fly AV checking (charitable explanation) or are too damn lazy to design their programs right (my explanation).
So if you are using a local app to check for compression, results will be subject to the vagaries of your computer setup. Better off to use a site designed for testing compression. Then you know what the server sends by default.
You can force gzip compression on the 10-20% who have it disabled b/c of their AV setup. This is what Google does. Essentially, you set a cookie, but the cookie data is gzipped. Then on subsequent page loads you see if the cookie is present. If it is, then ipso facto, the client can accept gzipped content regardless of how the headers are being mangled by Norton, and you send it to them, whether they asked for it or not.
If you have lots of traffic, that may be worth it (imagine the bandwidth Google saves by gzipping the 10% who could receive gzipped content but don't ask for it explicitly).