tedster - 8:11 pm on Mar 28, 2011 (gmt 0)
You use a server header checker utility. Many people use the LiveHTTPHeaders add-on for Firefox, but there are many other tools, too.
If Google is saying the URL gives them a 404 status, that is what is happening, at least for googlebot. You could even verify that within WMT by using the "Fetch as Googlebot" tool.