IIS: How to tell is a slow time-taken is due to a slow network connection

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http://serverfault.com – According to http://support.microsoft.com/kb/944884, "when a large response or large responses are sent to a client over a slow network connection, the value of the time-taken field may be more than expected". I have a situation where a client will say, "I sent a request to your web server at 10:03:24 and it took 20 seconds, why?". I can see this in the IIS logs as well, but the server's ASP.NET module logged it as taking 100ms, and CPU and Disk counters were low. I suspect that it's due to a slow network connection. How can I prove this? (HowTos)