It depends on what exactly you mean by "insertion". Do you mean a first-generation document, or a document that's new to the local database?
You can detect a new (first-generation) document by looking at the revision history, checking whether doc.currentRevision.parentRevision == nil. Or a cheaper way to do the same thing is to see whether the revision ID begins with "1-" which means it's first-generation.
But the replicator will add documents to the local database that aren't first-generation. You can check the `source` property of the CBLDatabaseChange to see whether this revision was added by the replicator, but there isn't anything that tells you whether the document existed locally beforehand.
—Jens