Are themes in use?

I have a bunch of themes that I've offered my users over the past two years. I'd like to begin deleting some as I add new ones.

Is there a quick way to see what themes are being used and what themes are not?

It would be ideal if there was just a big list of all my themes with numbers of instances of use next to each theme. Extending this idea, it would be nice to have a list of blogs using that theme (although this might now scale well) so that I could see if they are still in use or can be removed.