Below is the script for finding high water mark of a table.

--
-- Show the High Water Mark for a given table, or all tables if ALL is specified for Table_Name.
--

SET LINESIZE 300
SET SERVEROUTPUT ON
SET VERIFY OFF

DECLARE
CURSOR cu_tables IS
SELECT a.owner,
a.table_name
FROM all_tables a
WHERE a.table_name = Decode(Upper('&&Table_Name'),'ALL',a.table_name,Upper('&&Table_Name'))
AND a.owner = Upper('&&Table_Owner')
AND a.partitioned='NO'
AND a.logging='YES'
order by table_name;

op1 NUMBER;
op2 NUMBER;
op3 NUMBER;
op4 NUMBER;
op5 NUMBER;
op6 NUMBER;
op7 NUMBER;
BEGIN

Dbms_Output.Disable;
Dbms_Output.Enable(1000000);
Dbms_Output.Put_Line('TABLE UNUSED BLOCKS TOTAL BLOCKS HIGH WATER MARK');
Dbms_Output.Put_Line('------------------------------ --------------- --------------- ---------------');
FOR cur_rec IN cu_tables LOOP
Dbms_Space.Unused_Space(cur_rec.owner,cur_rec.table_name,'TABLE',op1,op2,op3,op4,op5,op6,op7);
Dbms_Output.Put_Line(RPad(cur_rec.table_name,30,' ') ||
LPad(op3,15,' ') ||
LPad(op1,15,' ') ||
LPad(Trunc(op1-op3-1),15,' '));
END LOOP;

END;