I have a couple of very old users in my NC, but they are no longer members of our club and therefore no longer use the NC. Before I delete their accounts now, I would like to see if there is still information for the club. Is it possible to look as admin or do I have to generate a new password to log in as user?
You can use the impersonate app.
If you are from Europe read General Data Protection Regulation. Maybe you can better first ask the user or delete without looking the data. Also you can make a backup, delete in Nextcloud and delete the backup data after a few years.