SQL> create function fun_test return boolean
  2  is
  3  begin
  4    return (true);
  5  end;
  6  /

Function created.

SQL> select * From dept where fun_test;
select * From dept where fun_test
                                *
ERROR at line 1:
ORA-00920: invalid relational operator

SQL> begin
  2    if fun_test then
  3       dbms_output.put_line('Function returns TRUE');
  4    end if;
  5  end;
  6  /
Function returns TRUE

PL/SQL procedure successfully completed.