How do I find my current SCN?
Current SCN

Oracle 9i:

SELECT dbms_flashback.get_system_change_number as current_scn 
FROM DUAL;


Oracle 10g and above:

SELECT current_scn
FROM V$DATABASE;


SCN Limits

SCN has a hard limit imposed by its format and a soft limit imposed artificially by Oracle, as described here. I've quoted the relevant portions below (emphasis added).

Hard Limit


  The architects of Oracle's flagship database application must have
  been well aware the SCN needed to be a massive integer. It is: a
  48-bit number (281,474,976,710,656). It would take eons for an Oracle
  database to eclipse that number of transactions and cause problems --
  or so you might think.


Soft Limit


  The soft limit derives from a very simple calculation anchored to a
  point in time 24 years ago: Take the number of seconds since 00:00:00
  01/01/1988 and multiply that figure by 16,384. If the current SCN
  value is below that, then all is well and processing continues as
  normal. To put this in simple terms, the calculation assumes that a
  database running constantly since 01/01/1988, processing 16,384
  transactions per second, cannot exist in reality.


SCN Limit Check

This script (Oracle 10g and above) will check how much of the hard and soft limits you have exhausted. Thanks to Rob for calling out the soft limit.

WITH limits AS (
  SELECT 
      current_scn
  --, dbms_flashback.get_system_change_number as current_scn -- Oracle 9i
    , (SYSDATE - TO_DATE('1988-01-01 00:00:00', 'YYYY-MM-DD HH24:MI:SS')) * 24*60*60 * 16384 
        AS SCN_soft_limit
    , 281474976710656 AS SCN_hard_limit
  FROM V$DATABASE
)
SELECT
    current_scn
  , current_scn/scn_soft_limit*100 AS pct_soft_limit_exhausted
  , scn_soft_limit
  , current_scn/scn_hard_limit*100 AS pct_hard_limit_exhausted
  , scn_hard_limit
FROM limits;