impersonate-users

Verify our current user and role

SELECT SYSTEM_USER
SELECT IS_SRVROLEMEMBER('sysadmin')
Go

Identify users

SELECT distinct b.name
FROM sys.server_permissions a
INNER JOIN sys.server_principals b
ON a.grantor_principal_id = b.principal_id
WHERE a.permission_name = 'IMPERSONATE'
GO

Impersonate the SA user

EXECUTE AS LOGIN = 'sa'
SELECT SYSTEM_USER
SELECT IS_SRVROLEMEMBER('sysadmin')
GO