determining 404

[Richard Gaskin]

When obtaining a web resource, if it's 404 the server will usually 
return a page noting that for the user.  That's all well and good, but 
the 404 pages I've seen appear to be designed for human reading, and do 
not appear to have any consistent elements which would allow a machine 
to identify that the resource was not found.

Is there something in the header that identifies that?

How can I know when a requested URL doesn't exist?

-- 
  Richard Gaskin
  Fourth World Media Corporation
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  Ambassador at FourthWorld.com       http://www.FourthWorld.com