It seems Dropbox has no easy way to allow the user to know how many files and folders does (s)he share with a particular person (for instance, by searching by a name or an e-mail). By now, it is possible only by checking manually one by one all the shared files and folders trying to find if a particular person is also sharing that file or folder with the user. I encourage the Dropbox Team to create a faster and easier way for checking it. Thanks!