What you can do is adding the repo workspace of the developer having delivered on the source stream.
You would add that repo workspace as a flow target of your own repo workspace (the one you are using for the merge)
By setting that new flow target as current, you can quickly see what change set you are missing.
Of course, the real issue is that what I just proposed doesn't scale: if you have multiple developers having made partial delivers (with missing intermediate change sets), then you would need to have a look in turn to each developer's repo workspaces.
That is why you have since 2007(!) the "blocker" Work Item "Enhancement 24822":
"Confirm Content Accept" dialog should be more specific (offer to fill in gaps when a gap exception occurs)
... but it is still unresolved, even in the latest RTC4.x.