http://stackoverflow.com/questions/786638/how-can-i-get-gzip-compression-in-iis7-working/787251
Look in c:inetpublogsFailedReqLogFilesw3svcx. You will see a bunch of files named fr000xx.xml. Open up any one of them in your browser. (By the way, if you copy these files anywhere, make sure freb.xsl is there. Also, don't delete freb.xsl - if you do, just delete the whole directory or copy it from another location, as IIS only creates it once per folder.)