Fiddler is the tool of choice when it comes to inspecting WCF messages that utilize an HTTP transport.  In a previous post I mentioned that HTTP WCF messages should be in encoded in a binary format to decrease their size.  The problem you run into when encoding your WCF messages is that when Fiddler attempts to inspect an encoded message, it’s not in a readable format.  To resolve this limitation, you can download a free plug-in which will add an inspector to Fiddler to decode binary WCF message.  Using this new inspector, you can inspect binary encoded WCF messages freely from within Fiddler.

http://code.msdn.microsoft.com/wcfbinaryinspector