Find Freespace in all the Tablespaces:

SELECT fs.tablespace_name name, df.totalspace mbytes, (df.totalspace - fs.freespace) used,
fs.freespace free, 100 * (fs.freespace / df.totalspace) pct_free
FROM (SELECT tablespace_name, ROUND(SUM(bytes) / 1048576) TotalSpace FROM dba_data_files GROUP BY tablespace_name ) df,
(SELECT tablespace_name, ROUND(SUM(bytes) / 1048576) FreeSpace FROM dba_free_space GROUP BY tablespace_name ) fs
WHERE df.tablespace_name = fs.tablespace_name(+);