transfer from desktop to endnote web

I currently have ~7000 references in my endnote web folder, but want to update the web folder w/ ~200 new references. I don’t know which specific references these are, and thought there would be some way for endnote to figure out which references are the same between the web and my machine, but am not finding this feature. Does anyone out there know how to do this? I would think this would be a common issue among many users, but maybe not.

Thanks for your time and input