Where can one find the high watermark for a table

There is no single system table, which contains the High Water Mark (HWM) for a table.

A table's HWM can be calculated using results from the following SQL statements 

SQL>SELECT BLOCKS
FROM DBA_SEGMENTS
WHERE OWNER = UPPER (owner) 
AND SEGMENT_NAME = UPPER (table);

ANALYZE TABLE owner.Table ESTIMATE STATISTICS; 

SQL>SELECT EMPTY_BLOCKS
FROM DBA_TABLES
WHERE OWNER = UPPER (owner) 
AND SEGMENT_NAME = UPPER (table); 

Thus, the tables HWM = (query result 1) - (query result 2) – 1

NOTE: You can also use DBMS_SPACE package and calculate

HWM = TOTAL_BLOCKS – UNUSED_BLOCKS – 1