See web server response headers using firebug in firefox or developer tools in chrome:
If resource is gzipped you will see header:
Content-Encoding:gzip
You can also use any CLI web clients, e.g. wget:
wget -S -O /dev/null http://72.10.33.203/workspace/js/plugins.min.js
It doesn't show content-encoding, but if you use
wget --header='Accept-Encoding:gzip,deflate,sdch' -S -O /dev/null`
Connecting to 72.10.33.203:80... connected.
HTTP request sent, awaiting response...
HTTP/1.1 200 OK
Date: Fri, 18 May 2012 19:51:12 GMT
Server: Apache
Last-Modified: Fri, 18 May 2012 19:42:04 GMT
Accept-Ranges: bytes
Cache-Control: max-age=31536000
Expires: Sat, 18 May 2013 19:51:12 GMT
Vary: Accept-Encoding
Content-Encoding: gzip <================================ Look here
X-Powered-By: PleskLin
Connection: close
Transfer-Encoding: chunked
Content-Type: application/javascript; charset=utf-8