I don’t think there is a way to know with the current Android version. With the desktop version, you can launch the Qt GUI with F5 and there will be a counter in the left corner.
If you are worried about missing some games with the internal scan function you can use the online RetroArch PlayList Editor. You can then drag & drop a RetroArch Database RDB file to identify each game (especially useful for MAME).
You will also need to know your internal memory/external SD mount point in Android and edit your paths. It is a pretty powerful tool (though not quite there yet for CD based games). Make sure to take a look at the README on how to use.
You can also create playlists with the Desktop GUI and drag & dropping the contents of a directory in the main window. Then just batch edit the paths with a powerful text editor such as Notepad++.