How do I search for templates that aren't assigned to any files? I'd like to get a list to review and delete the ones we don't use.
Now that you mention it, I'm not aware of a way to identify "unused" templates from the Admin console and that seems like it would be a useful feature to add for maintenance purposes.
The admin console does have a kind of safety feature to check for entries using a template before it will delete to prevent that, but not sure about a way to actively "find" them.
Normally I wouldn't recommend going directly to the database, but unless there's a way that I'm missing you could get a list with the following query.
SELECT * FROM [propset] WHERE [pset_id] NOT IN ( SELECT DISTINCT [pset_id] FROM [toc] WHERE [pset_id] IS NOT NULL )