How to test IIS Compression?

You can test your server's compression here.

There is a nice free tool for Internet Explorer which can be used to see if your server's http responses are compressed:

Fiddler

Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP(S) traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler includes a powerful event-based scripting subsystem, and can be extended using any .NET language.

Fiddler is freeware and can debug traffic from virtually any application, including Internet Explorer, Mozilla Firefox, Opera, and thousands more.

Here is a nice tutorial: "Fiddler Can Make Debugging Easy."

Fiddler and the link above work well. In fact, fiddler is often the best option. However, if you want an IE plug-in directly, here's a great link with various options: http://blog.port80software.com/2009/08/17/tap-tap-is-this-thing-on/.