Is there anyway to determine on Page Load if the URL is the Original or the Rewritten one?  I've played with several of the Request.URL properties but nothing seems to determine which URL is being used.

It looks like the Search Engines are using my original URL's (when they spider me) and I would like a way for my pages to do a 301 Redirect if the original URL is used. 

 something like

 

If (orignal URL) then

Response.Status = "301 Moved Permanently"

Response.AddHeader("Location", "http://A_Re_Written_URL")

End If

 Thank you.

Just don't show out your original URL-s to search engines. They refresh their indexes during time and this doen't happen over night. Original URL you have may contain specific query string parameter by example. You can do your own redirect by example in BeginRequest event of your application. Of course, be sure you end response right after redirect.

When you rewrite your urls in your handler you can add a query param rewrite=true. no one will see this but if its not there you can forward to your friendly URL. Another option is to add a boolean session variable.

The issues with the whole 301 thing is that you are sending the person to the same page using a different url. If something goes wrong in your code you will get an unending loop of page redirects.