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.

DECLARE   @UserOptions TABLE(SetOption varchar(100), Value varchar(100))
DECLARE   @IsolationLevel varchar(100)

INSERT    @UserOptions
EXEC('DBCC USEROPTIONS WITH NO_INFOMSGS')

SELECT    @IsolationLevel = Value
FROM      @UserOptions
WHERE     SetOption = 'isolation level'

-- Do whatever you want with the variable here...  
PRINT     @IsolationLevel

If you are talking about the current transaction nesting level, then you would use @@TRANCOUNT.

If you are talking about transaction isolation level, use DBCC USEROPTIONS and look for an option of isolation level. If it isn't set, it's read committed.

SELECT CASE  
    WHEN transaction_isolation_level = 0 THEN 'Unspecified' 
    WHEN transaction_isolation_level = 1 THEN 'Read Uncommitted' 
    WHEN transaction_isolation_level = 2 AND d.snapshot_isolation_state_desc = 'OFF' THEN 'Read Committed' 
    WHEN transaction_isolation_level = 2 AND d.snapshot_isolation_state_desc = 'ON' AND d.is_read_committed_snapshot_on = 1 THEN 'Snapshot Read Committed'  
    WHEN transaction_isolation_level = 2 AND d.snapshot_isolation_state_desc = 'ON' AND d.is_read_committed_snapshot_on = 0 THEN 'Snapshot'  
    WHEN transaction_isolation_level = 3 THEN 'Repeatable Read' 
    WHEN transaction_isolation_level = 4 THEN 'Serializable' END AS TRANSACTION_ISOLATION_LEVEL,
    d.is_read_committed_snapshot_on,
    d.snapshot_isolation_state_desc
FROM sys.dm_exec_sessions 
       CROSS JOIN sys.databases AS d
where session_id = @@SPID
  AND  d.database_id = DB_ID();