2 answers
Stefanie - do you mean public groups as in Chatter groups that are public, or public groups as in groups of users created through Setup > Manage Users > Public Groups?
If it's the latter, I don't know of one location to check where they're all being used. This app *may* get you somewhere near it, perhaps: https://appexchange.salesforce.com/listingDetail?listingId=a0N300000016a75EAA. Otherwise one of the more developer-y tools like the explorer or metadata API should help out I guess.