Oracle SecureFile Compression
Assuming you have the appropriate DBA-level permissions, you can try this to get some basic information on the SecureFile/LOBs in your system:

SELECT table_name, segment_name, index_name, securefile, in_row
FROM user_lobs;


You can get the size on disk for a specific table name (your table with a SecureFile/LOB) by doing this:

SELECT segment_name, segment_type, segment_subtype, bytes/1024/1024 meg_bytes
FROM user_segments
WHERE table_name = 'yourTableName';


To get the actual compression rating (no/low/high), try this using your table name:

SELECT table_name, column_name, partition_name, tablespace_name, compression, deduplication
FROM user_lob_partitions
WHERE table_name = 'yourTableName';


Hope this helps!