Once you have the backup restored and you’ve managed to log in as an admin, you should be able to find the plugins you were using by inspecting the site settings json file.
Open [YourSite]/admin/site_settings.json and search for "plugin":. You should find things like "plugin":"discourse-narrative-bot" which will tell you that you were using the discourse-narrative-bot plugin. That’s a built-in one so you can ignore that along with poll, styleguide, discourse-presence, discourse-local-dates and discourse-details.
Note that plugins you installed in the past and later removed will also be there so this won’t necessary make a list of all the plugins you were using at the time of creating the backup, rather all the plugins you had used in the history of that site.