There's a java method getSrcAppName (see below) and there's also a variable you can access in the decision editor in Studio under the Call Data tab (Source App Name) that is blank if it's a new call, otherwise has the app name.
<pre>public
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html getSrcAppName()</pre>
<dl><dd>This method returns the name of the application that performed an application transfer to
reach the current application. If the physical call began with this application, the method
returns a <code>null</code>.
</dd><dd>
</dd><dd><dl><dt>
Returns:</dt><dd>The name of the application that performed an application transfer to this
application, or <code>null</code> if the caller called directly into this
application.</dd></dl>
</dd></dl>