The best approach is probably to wait till the pull replication finishes or goes idle, check whether it pulled a nonzero number of docs, and then run an all-docs query with its allDocsMode set to kCBLOnlyConflicts.
Another approach is to observe the kCBLDatabaseChangeNotification using NSNotificationCenter. The userInfo of the notification contains an array of CBLDatabaseChange objects. Each of those has an .inConflict property that indicates whether the doc is now in conflict.
—Jens