404 Error "Standards"

In the process of researching how to "ping" a URL to check its validity, a
quick solution of just checking for a string involving "HTTP 404 Error" and
variations of it presented itself as a quick solution.  Is there currently a
"default" standard for web servers for 404 errors?  Is there a specific HTTP
header or META tag I can look for in the page that gets returned when the
page does not exist on the server? (This would probably be more consistent -
if it exists)

Any advice here would be great!!!

Thanks,

David Siegel
mailto:david.siegel@dialogic.com