The following query from the same session that performs "alter controlfile to trace" will return the name of the trace file created. Of course it'll be put into your udump directory.

select 'ORA' || ltrim(to_char(p.spid,'09999')) || '.TRC' as trace_file
from v$process p, v$session s
where p.addr = s.paddr
and s.sid = (select sid from v$mystat where rownum=1);

P.S.
If you are using 9i and you are doing "backup controlfile to trace" from inside oracle's DBMS_JOB scheduler, the abowe query is useles, as in 9i each queue job writs its trace information into its separate, dedicated trace file named 'J00n.TRC'. I haven't found out yet how to determine the number "n" from inside the scheduled job. If anyone knows I'd be very glad to hear it.