Pretty straightforward, check if dbms_transaction.step_id is null!
SQL> select dbms_transaction.step_id from dual;
STEP_ID
---------------
SQL> insert into t values (1);
1 row created.
SQL> select dbms_transaction.step_id from dual;
STEP_ID
---------------
114352430549782
SQL> commit;
Commit complete.
SQL> select dbms_transaction.step_id from dual;
STEP_ID
---------------
SQL> insert into t values (2);
1 row created.
SQL> select dbms_transaction.step_id from dual;
STEP_ID
---------------
124248035235852
SQL> rollback;
Rollback complete.
SQL> select dbms_transaction.step_id from dual;
STEP_ID
---------------