How to tell which purchased themes have not been used

Is there a way to know which themes have been used? For example, if I purchased 10 themes and have only used 2 - how can I tell which ones have not been used?

Alternatively, is there a way to take the license key from a website and know which download it goes to? I want to know this so I can download the right theme files for that client.

You can contact the item authors for the support.

Can you further explain what you mean because I am new to this

There is no such features and you have to make your own note for this. some author provide their own support forum where a feature exist to see your used licenses and domains list.