In /user/register just try to create a username and if the name is already taken it will be notified:
If you request a new password for an existing username:
If you request a new password for a non-existent username:
Accessing /user/<number> you can see the number of existing users, in this case is 2 as /users/3 returns a not found error:
$ is a number (from 1 to 500 for example).
You could find hidden pages (test, dev) which are not referenced by the search engines.
You need the plugin php to be installed (check it accessing to /modules/php and if it returns a 403 then, exists, if not found, then the plugin php isn't installed)
Go to Modules -> (Check) PHP Filter -> Save configuration
Then click on Add content -> Select Basic Page or Article -> Write php shellcode on the body -> Select PHP code in Text format -> Select Preview