return all empty folders in a document library using webservice

For a client I’m looking at the following case:

They have all sorts of documents within a document library grouped into folders (multiple levels deep). If a folder doesn’t contain a document, the process owner knows that something is wrong since it’s not complete. They want to be able to manually start a workflow to check a particular folder for completeness. If it’s empty an email must be send with the name of the folder.

I was thinking of using SharePoint Online 2013 workflow however there is no action available to check if a folder is empty or not. So my second thought was using a webservice to do it for me, but I need some guidance on how to do that.

• Can this be done by leveraging the default web services from SP2013 Online?
• How can I check if a folder is empty?

I am not a programmer, but I know how to write some code. Any suggestions are welcome.

Thanks,
Kevin