FAQ: How to find the authentication mode of a SQL Server Instance ?

Using System Stored procedure xp_loginconfig

xp_loginconfig system strored procedure will give you the authentication information in SQL Server. You can call the sp without parameter to get detailed information or with “Loging Mode” parameter to get only the Authentication mode.
EG.
EXEC xp_loginconfig 'login mode'

Result
Name Config_Value
login mode Mixed

Read Registry to get the authentication Information.

Another method would be reading Registry. Authentication information can be read from the following registry entry.
HKEY_LOCAL_MACHINE\Software\Microsoft\Microsoft SQL Server\MSSQL.##\MSSQLServer\LoginMode

Note : Change ## the number according to your environment.

Eg.

EXECUTE MASTER..XP_REGREAD
'HKEY_LOCAL_MACHINE', 'SOFTWARE\MICROSOFT\MICROSOFT SQL SERVER\MSSQL.4\MSSQLSERVER',
'LOGINMODE'
Result
Value Data
LoginMode 1