T-SQL: check replication backlog

You can use the code below on your distributor in order to check the backlog of the replication

SELECT 
(CASE  
    WHEN mdh.runstatus =  '1' THEN 'Start - '+cast(mdh.runstatus as varchar)
    WHEN mdh.runstatus =  '2' THEN 'Succeed - '+cast(mdh.runstatus as varchar)
    WHEN mdh.runstatus =  '3' THEN 'InProgress - '+cast(mdh.runstatus as varchar)
    WHEN mdh.runstatus =  '4' THEN 'Idle - '+cast(mdh.runstatus as varchar)
    WHEN mdh.runstatus =  '5' THEN 'Retry - '+cast(mdh.runstatus as varchar)
    WHEN mdh.runstatus =  '6' THEN 'Fail - '+cast(mdh.runstatus as varchar)
    ELSE CAST(mdh.runstatus AS VARCHAR)
END) [Run Status], 
mda.subscriber_db [Subscriber DB], 
mda.publication [PUB Name],
right(left(mda.name,LEN(mda.name)-(len(mda.id)+1)), LEN(left(mda.name,LEN(mda.name)-(len(mda.id)+1)))-(10+len(mda.publisher_db)+(case when mda.publisher_db='ALL' then 1 else LEN(mda.publication)+2 end))) [SUBSCRIBER],
CONVERT(VARCHAR(25),mdh.[time]) [LastSynchronized],
und.UndelivCmdsInDistDB [UndistCom], 
mdh.comments [Comments], 
'select * from distribution.dbo.msrepl_errors (nolock) where id = ' + CAST(mdh.error_id AS VARCHAR(8)) [Query More Info],
mdh.xact_seqno [SEQ_NO],
(CASE  
    WHEN mda.subscription_type =  '0' THEN 'Push' 
    WHEN mda.subscription_type =  '1' THEN 'Pull' 
    WHEN mda.subscription_type =  '2' THEN 'Anonymous' 
    ELSE CAST(mda.subscription_type AS VARCHAR)
END) [SUB Type],

mda.publisher_db+' - '+CAST(mda.publisher_database_id as varchar) [Publisher DB],
mda.name [Pub - DB - Publication - SUB - AgentID]
FROM distribution.dbo.MSdistribution_agents mda 
LEFT JOIN distribution.dbo.MSdistribution_history mdh ON mdh.agent_id = mda.id 
JOIN 
    (SELECT s.agent_id, MaxAgentValue.[time], SUM(CASE WHEN xact_seqno > MaxAgentValue.maxseq THEN 1 ELSE 0 END) AS UndelivCmdsInDistDB 
    FROM distribution.dbo.MSrepl_commands t (NOLOCK)  
    JOIN distribution.dbo.MSsubscriptions AS s (NOLOCK) ON (t.article_id = s.article_id AND t.publisher_database_id=s.publisher_database_id ) 
    JOIN 
        (SELECT hist.agent_id, MAX(hist.[time]) AS [time], h.maxseq  
        FROM distribution.dbo.MSdistribution_history hist (NOLOCK) 
        JOIN (SELECT agent_id,ISNULL(MAX(xact_seqno),0x0) AS maxseq 
        FROM distribution.dbo.MSdistribution_history (NOLOCK)  
        GROUP BY agent_id) AS h  
        ON (hist.agent_id=h.agent_id AND h.maxseq=hist.xact_seqno) 
        GROUP BY hist.agent_id, h.maxseq 
        ) AS MaxAgentValue 
    ON MaxAgentValue.agent_id = s.agent_id 
    GROUP BY s.agent_id, MaxAgentValue.[time] 
    ) und 
ON mda.id = und.agent_id AND und.[time] = mdh.[time] 
where mda.subscriber_db<>'virtual' -- created when your publication has the immediate_sync property set to true. This property dictates whether snapshot is available all the time for new subscriptions to be initialized. This affects the cleanup behavior of transactional replication. If this property is set to true, the transactions will be retained for max retention period instead of it getting cleaned up as soon as all the subscriptions got the change.
--and mdh.runstatus='6' --Fail
--and mdh.runstatus<>'2' --Succeed
order by mdh.[time]