Is there anyway to say whether gzip really compresses certain files?

Question

I know that gzip is running on my server and site. Most of all this will be taken care of, but when I run firebug or google pagespeed, I am told that some CSS files (posted by me) are not gzipped and can be very compressed.

Any thoughts are appreciated.

Update

I was asked about my server: MediaTemple DV 3.5, working with CentOS 5 with PHP 5.3.2 and Apache 2.2.15

Answer 1

FireBug is great for this purpose. Just go to your website and on the Net tab, check the response headers for all requests. For example, gzipped CSS is correct here (on StackOverflow, of course):

You can also install the YSlow plugin and it will tell you exactly what static resources can be compressed to reduce throughput.