Use dbms_space.unused_space to find unused space, whose arguments are:
   segment_owner              IN  VARCHAR2, 
   segment_name               IN  VARCHAR2,
   segment_type               IN  VARCHAR2,
   total_blocks               OUT NUMBER,
   total_bytes                OUT NUMBER,
   unused_blocks              OUT NUMBER,
   unused_bytes               OUT NUMBER,
   last_used_extent_file_id   OUT NUMBER,
   last_used_extent_block_id  OUT NUMBER,
   last_used_block            OUT NUMBER, 
   partition_name             IN  VARCHAR2 DEFAULT NULL);

SQL> var tb number
SQL> var tby number
SQL> var ub number
SQL> var uby number
SQL> var l1 number
SQL> var l2 number
SQL> var l3 number
SQL> exec dbms_space.unused_space('R_STAGE1','EB_GL_TRANSACTIONS','TABLE',:tb,:tby,:ub,:uby,:l1,:l2,:l3)

PL/SQL procedure successfully completed.

SQL> print

        TB
----------
   1721600


       TBY
----------
2.8207E+10


        UB
----------
         0


       UBY
----------
         0


        L1
----------
        38


        L2
----------
    129925


        L3
----------
       640

So in this case, HMW is in the last block (ub or unused blocks and uby or unused bytes are 0).

To find where HMW is located physically,

SQL> select header_file, header_block from dba_segments where segment_name = 'EB_GL_TRANSACTIONS';

HEADER_FILE HEADER_BLOCK
----------- ------------
         26         3205

SQL> alter system dump datafile 26 block 3205;

System altered.

In the dump file, we see

  Extent Control Header
  -----------------------------------------------------------------
  Extent Header:: spare1: 0      spare2: 0      #extents: 2690   #blocks: 1721598
                  last map  0x0ac0b405  #maps: 1      offset: 8224
      Highwater::  0x0981fe05  ext#: 2689   blk#: 640    ext size: 640
  #blocks in seg. hdr's freelists: 1406670
  #blocks below: 1721598
  mapblk  0x0ac0b405  offset: 1672
                   Unlocked
     Map Header:: next  0x0ac0b405  #extents: 1017 obj#: 38223  flag: 0x40000000
  Extent Map
  -----------------------------------------------------------------
   0x06800c86  length: 639
   0x08c00005  length: 640
   0x09000005  length: 640
   0x09400005  length: 640
...
   0x0a40b405  length: 640
   0x0a80b405  length: 640

  nfl = 1, nfb = 1 typ = 1 nxf = 0 ccnt = 344324
  SEG LST:: flg: USED   lhd: 0x0bc1016a ltl: 0x0981fe04
End dump data blocks tsn: 28 file#: 26 minblk 3205 maxblk 3205

The address of HMW is 0x0981fe05, or 159514117. Find the location:

SQL> var a number
SQL> exec :a := dbms_utility.data_block_address_file(159514117)

PL/SQL procedure successfully completed.

SQL> print a

         A
----------
        38

SQL> exec :a := dbms_utility.data_block_address_block(159514117)

PL/SQL procedure successfully completed.

SQL> print a

         A
----------
    130565

SQL> select * from dba_extents where file_id = 38 and segment_name = 'EB_GL_TRANSACTIONS' order by extent_id;

OWNER                          SEGMENT_NAME                                                              PARTITION_NAME                 SEGMENT_TYPE
------------------------------ --------------------------------------------------------------------------------- ------------------------------ ------------------
TABLESPACE_NAME                 EXTENT_ID    FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
------------------------------ ---------- ---------- ---------- ---------- ---------- ------------
R_STAGE1                       EB_GL_TRANSACTIONS                                                                                       TABLE
R_STAGE_DATA                            4         38          5   10485760        640           38

...

R_STAGE1                       EB_GL_TRANSACTIONS                                                                                       TABLE
R_STAGE_DATA                         2689         38     129925   10485760        640           38

The last block is 129925 + 640 = 130565, matching HWM.