Being a DBA you would like to have information related to access at various levels in SQL Server. Primarily the question is that who can login to your SQL Server instance. Use following command to get list of all log ins who are able/disable to log in to your SQL Server instance. Command will query sys.server_principals system view and will get information about windows logins and SQL Server logins.
SELECT name, type_desc, is_disabled FROM sys.server_principals
Result will be in following form
Also you may apply filter on ‘isdisabled‘ and ‘type_desc‘. In above result set login name sa is disabled so it can not be used for login to SQL Server instance.