IIS7.0 Failed Request Tracing (FRT) is a powerful troubleshooting tool for request-processing failures. More details regarding FRT and how to enable it can be be found here.
Once the failed request tracing is enabled for your site, do the following to trace rewrite rules:
After the FRT rule is created, the log will start getting generated in %SystemDrive%\inetpub\Logs\FaileReqLogFiles\. You can open this log by using Internet Explorer, and it will be rendered as an HTML document that can be easily browsed.
PS: If the Failed Request Tracing was installed after URL rewrite module, the "Rewrite" area in Trace Providers may not be available. If you do not see "Rewrite" area listed there, go to Add/Remove programs and then run URL rewrite module installer in repair mode.
For more details on this, read this walkthrough.
SDET, IIS Team