When I add a new ePub to my library, I check that the metadata Calibre will use in its database is OK. I also like to embed the metadata in the ePub file so I use the Embed Metadata plug-in -- in other words, I click on that button in the toolbar. When I do that, there's no message confirming that the embedding was successful. Am I missing something? I've spot checked this a couple times by looking inside the ePub file at the metadata before and after clicking the button -- and sure enough it seems to work -- the metadata is indeed added to the file. However, I'd feel more comfortable if there was a confirmation message.
I also use the Embed Comic Metadata plug-in. It pops up a message telling me that the embedding was successful and that gives me a warm, fuzzy feeling. Like I say, maybe there's also an indicator in Embed Metadata, but I'm not spotting it.
BTW, I'm a huge Calibre fan. Love it and use it every day! Wish I would have started using it years ago.