SQL Server 2014 introduced sys.column_store_row_groups system view for finding the information on the Columnstore row groups.

Using this system view, you can find information like total number of rows physically stored, and a column for the number of rows marked as deleted.

If you find a high volume of deleted rows, you should decide to rebuild it.

T-SQL Script:

SELECT
	OBJECT_NAME([csrg].[object_id]) AS TableName
	,[i].[name] AS IndexName
	,[csrg].[row_group_id]
	,[csrg].[delta_store_hobt_id]
	,[csrg].[state_description]
	,[csrg].[total_rows]
	,[csrg].[deleted_rows]
	,[csrg].[size_in_bytes]
FROM sys.column_store_row_groups AS csrg
LEFT OUTER JOIN sys.indexes AS i
	ON [csrg].[object_id] = [i].[object_id]
		AND [csrg].[index_id] = [i].[index_id]
ORDER BY [TableName],[IndexName]
GO