Filegroup Usage

SELECT OBJECT_NAME(p.object_id) AS table_name,
       p.index_id,
       p.rows,
       au.type_desc
  FROM sys.partitions AS p
 INNER JOIN sys.allocation_units AS au ON p.hobt_id = au.container_id
 INNER JOIN sys.filegroups AS f ON au.data_space_id = f.data_space_id
 WHERE f.name = 'YourFilegroupName'