I believe whether or not a response is compressed for IIS depends on the following:
1) The mime type of the data being returned. This can be configured in IIS to allow/remove more types.
2) The accept-encoding header of the requestor. For the response to be compressed the accept-encoding header usually will contain gzip, deflate, or both.
3) The number of times the resource is requested. By default a resource needs to be request at a certain frequency in IIS before it will be compressed. See http://stackoverflow.com/questions/2203798/in-iis7-gzipped-files-do-not-stay-that-way.

To verify that the response is compressed, you need to look at the content-encoding header of the response. This is viewable from the network tab of the developer tools for most browsers. Compressed responses should contain gzip or deflate for that header value.

See http://www.iis.net/configreference/system.webserver/httpcompression for more information.