you cannot, not until 11g Release 1 anyway, in plsql.

You would have to use a client external from the database. Any external client like java/jdbc, C, C++, visual basic, etc - would do this the way they describe any cursor.

In 11g Release 1 and above, you have dbms_sql.

ops$tkyte%ORA11GR1> declare
  2          l_rcursor sys_refcursor;
  3          l_colCnt number;
  4          l_descTbl dbms_sql.desc_tab;
  5  begin
  6          open l_rcursor for select * from all_users;
  7
  8          dbms_sql.describe_columns
  9          ( c       => dbms_sql.to_cursor_number(l_rcursor),
 10            col_cnt => l_colCnt,
 11            desc_t  => l_descTbl );
 12
 13          for i in 1 .. l_colCnt
 14          loop
 15                  dbms_output.put_line( l_descTbl(i).col_name );
 16          end loop;
 17  end;
 18  /
USERNAME
USER_ID
CREATED

PL/SQL procedure successfully completed.

but not before.