[Zope] <dtml-if url type contruct>

What would be the correct dtml-if contrstruct for this behavior? I want to check and see if zope can resolve a url to a valid object. Ie does http://foo.com/bar/foo get you to a valid page or is it not found. I need something like <dtml-if http://foo.com/bar/foo> that way I can know when to rewrite the url based on zope finding the object or not. Thanks Kosh