As far as I know, you can't. The contents of templates are added to the page's source after the page is first saved, so there's really nothing that indicates it was originally built from a template other than the content itself.
If there's something unique about your template (say, the presence of a particular div's class or maybe a macro that you don't use anywhere else), then you could search the database for the presence of that value. If this is a possibility, then you'll want to search the BODYCONTENT table's BODY field for that value.
Otherwise, I can't think of how to do what you're asking.
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