Discovering which themes are being used In a Multisite installation

How can I discover which themes are being used In a Multisite installation?

I have a bunch of subdomain sites, and a bunch of themes installed, some of the themes are being used, and some are not. … I want to delete a bunch of those unused/old themes.

How can I discover if the theme is being used by any of the installations subdomain sites before I delete something that may be in use?