How to tell if mod_deflate is actually working?

What do the headers tell you, if it is not returning "content-encoding: gzip" it's probably not working.. you can test as follows:

curl -I -H 'Accept-Encoding: gzip,deflate' http://yoursite.com/somefile

The apache docs for AddOutputFilterByType indicate this directive is deprecated in Apache httpd 2.1 and later and it doesn't always work well if Apache can not determine the mime type.

I would suggest enabling compressing by using something like the following as a starting point and then add all the browser tweaks and compression levels back in. Obviously, you may want to double check httpd.conf to make sure it's actually loading mod_deflate.so as well:

<Location />
    SetOutputFilter DEFLATE
    # Don't compress images
    SetEnvIfNoCase Request_URI \.(?:gif|png|jpg|jpeg)$ no-gzip dont-vary
    Header append Vary User-Agent env=!dont-vary
</Location>

Use cURL as mentioned by Michael Steinfeld to verify.