How to verify which scheme SQL Server is using to authenticate a client?

While SQL Server authenticates a user, it uses specific authentication schemes (eg. Kerberos or NTLM or SQL) based on various factors.  We can use below code to figure out how a client (essentially SPID) is actually authenticated with SQL Server:

select auth_scheme
from sys.dm_exec_connections where
session_id= [SPID]

Note: As this is a DMV, it will work only on SQL 2005 onwards and you need to have atleast VIEW SERVER STATE Permission.