SELECT MAX(CAST(LTRIM(CAST(subID AS VARCHAR(MAX))) AS bigint)) AS id
FROM 
    (
    SELECT
        $PARTITION.YourPatitionFunction(YourPartitionColumn) AS [partitionNum]
        , MAX(id) AS [subID]
    FROM tbl_test 
    GROUP BY $PARTITION.YourPatitionFunction(YourPartitionColumn)
    ) AS a

declare @numrows int
declare @return_value int	

-- Get number of rows in table
SELECT @numrows=sum(p.rows) FROM sys.partitions p
inner join sys.indexes i on p.object_id=i.object_id and p.index_id=i.index_id
WHERE p.object_id=OBJECT_ID('TBL_TEST')
and name='PK_TBL_TEST_IDENTITY_ID'

-- Get max value of identity column
select max(IDENTITY_ID) from TBL_TEST 
where IDENTITY_ID >= @numrows