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
  )