How to find current transaction level?

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.

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

SELECT CASE  
          WHEN transaction_isolation_level = 1 
             THEN 'READ UNCOMMITTED' 
          WHEN transaction_isolation_level = 2 
               AND is_read_committed_snapshot_on = 1 
             THEN 'READ COMMITTED SNAPSHOT' 
          WHEN transaction_isolation_level = 2 
               AND is_read_committed_snapshot_on = 0 THEN 'READ COMMITTED' 
          WHEN transaction_isolation_level = 3 
             THEN 'REPEATABLE READ' 
          WHEN transaction_isolation_level = 4 
             THEN 'SERIALIZABLE' 
          WHEN transaction_isolation_level = 5 
             THEN 'SNAPSHOT' 
          ELSE NULL
       END AS TRANSACTION_ISOLATION_LEVEL 
FROM   sys.dm_exec_sessions AS s
       CROSS JOIN sys.databases AS d
WHERE  session_id = @@SPID
  AND  d.database_id = DB_ID();