To dump an undo segment header use the command function
ALTER SYSTEM DUMP UNDO_HEADER 'segment_name';
In Oracle 9.0.1 and above, if system managed undo is enabled, the segment name has to be in double quotes and upper case. This is because there is a leading underscore in the segment name.
A list of undo segment IDs and names can be obtained using
SELECT segment_id, segment_name FROM dba_rollback_segs ORDER BY segment_id;