There may be a couple of approaches to consider. First is the language construct Utilities.ExecutionSource/ExecutionSourceInfo as documented the Formativ Language Guide. This command enables you to determine the type of control which initiated the applet. The problem with it for your needs is that it will identify applet execution from the toolbar but won’t distinguish between the GroupWise client’s main toolbar and a message view’s toolbar.
The other option is to use the Applet Chaining feature along with Utilities.TransferData as a parameter. You can write a separate stub applet for each view integration you wish to identify then, using TransferData, pass the desired context and any other information to a separate main applet, whose behavior changes according to the applet which initiated it.
Hope this helps and provides some avenues for investigation.