How to know the source document for pasted page items

Hi,

I want to perform some actions on items before they are pasted (look at persistent data stored on the, if a conflict ask the user and either update or reject).

Believe I'm OK with the paste (document observer, in kBeforeDoMessageBoss where the change is kPasteCmdBoss, set global error code to kCancel to prevent the drop)

But... I can't think of any way to find the source document since the paste is from the scrap db.  I've tried kCopyCmdBoss in the observer but that isn't hit (maybe I need to observe the scrap?).

Hope someone can help.

TIA Ian