Prior versions as a DBA we used
to check the alert logs in background_dump_dest directory. But in 11g onwards the alert log file location is changed to different location mentioned below. The second option is adrci which is the easiest way to access the alert log files for the databases which are running on the box.
Location is : $ORACLE_BASE/diag/ rdbms/SID/SID/trace
ADRCI: Release 220.127.116.11.0 - Production on Wed Jun 24 00:07:59 2015
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ADR base = "/apps2/oracle"
adrci> show alert
Choose the home from which to view the alert log:
Q: to quit