Using sqlplus, simply type
"archive log list"
Typical output when OFF:
Database log mode No Archive Mode
Automatic archival Enabled
Archive destination /oracle/app/oracle/oradata/RALEPH/archive
Oldest online log sequence 305
Current log sequence 307
Typical output when ON:
Database log mode Archive Mode
Automatic archival Enabled
Archive destination /oracle/app/oracle/oradata/EXTENS/archive
Oldest online log sequence 193
Next log sequence to archive 195
Current log sequence 195