Editing for clarity:
When searching for donors in Civi when you mouse over the donor icon in the search results it will show you "last donated" with a date and amount. Unless the donor donated to the endowment in which case it will show nothing. So when trying to merge CID's, we must go into the individual donation record to check it, as it is not clear when just mousing over.
CID example to search by email: 47906217.