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This just came in via Twitter so I thought I'd share the tip here. @orangeexception asked how you could tell if a file was being run as a custom tag. One quick and dirty way to do this is to just check for the existence of the ThisScope scope:
<cfif structKeyExists(variables, "ThisTag")> custom tag <cfelse> not a custom tag </cfif>
I've never bothered to do this with a custom tag, but I can see how it could help enforce folks using your tags in the right way.