SQL> select log_mode from v$database;

LOG_MODE
------------
ARCHIVELOG

1 row selected.

or if you are in ARCHIVELOG mode, you will see rows returned by this ps command:
$ ps -ef |grep -i _arc

Change to NO-ARCHIVE logmode

SQL> connect / as sysdba;
SQL> shutdown immediate;
SQL> startup mount;
SQL> alter database noarchivelog;
Database altered.
SQL> alter database open;