If you are looking for a function like ifDefined for ColdFusion, you won't find it! Its not called IfDefined, its called isDefined. Here's an example code:

<cfif isDefined("Session.login") and EmptySession eq 0 >
  ...
<cfelse>
...
</cfif>

Incidently the above code is one way to reset a ColdFusion session. You should experiment to what works best in your enviroment!