Originally posted by rajabalachandra
select sum(bytes)/1024/1024/1024 ||'Gb' from dba_extents where segment_name = 'XXX';

this shows the extents allocated to the table.

is there any way to find how much of the extents
are occupied with rows?
Nope. At least not without trace-dumping all the blocks belonging to the particular segment and inspecting those dumps.