Checking The Cache Hit Ratio

   SELECT (P1.value + P2.value - P3.value) / (P1.value + P2.value)
   FROM   v$sysstat P1, v$sysstat P2, v$sysstat P3
   WHERE  P1.name = 'db block gets'
   AND    P2.name = 'consistent gets'
   AND    P3.name = 'physical reads';

http://www.dbspecialists.com/files/presentations/buffercache.html

SELECT name, physical_reads, db_block_gets, consistent_gets,
       1 - (physical_reads / (db_block_gets + consistent_gets)) "Hit Ratio"
  FROM V$BUFFER_POOL_STATISTICS;

http://docs.oracle.com/cd/E16655_01/server.121/e15857/tune_buffer_cache.htm#TGDBA545