Solution 1:

Run this:

SELECT CASE transaction_isolation_level 
WHEN 0 THEN 'Unspecified' 
WHEN 1 THEN 'ReadUncommitted' 
WHEN 2 THEN 'ReadCommitted' 
WHEN 3 THEN 'Repeatable' 
WHEN 4 THEN 'Serializable' 
WHEN 5 THEN 'Snapshot' END AS TRANSACTION_ISOLATION_LEVEL 
FROM sys.dm_exec_sessions 
where session_id = @@SPID

docs.microsoft.com reference for the constant values.

Solution 2:

just run DBCC useroptions and you'll get something like this:

Set Option                  Value
--------------------------- --------------
textsize                    2147483647
language                    us_english
dateformat                  mdy
datefirst                   7
lock_timeout                -1
quoted_identifier           SET
arithabort                  SET
ansi_null_dflt_on           SET
ansi_warnings               SET
ansi_padding                SET
ansi_nulls                  SET
concat_null_yields_null     SET
isolation level             read committed