monitoring bdr nodes

I need some clarification on how to monitor BDR nodes.  In particular determining replication lag.  As an example,  I
havea two node cluster with nodes ‘A’ and ‘B’.    I need to be able to look at node ‘B’ and determine if it is lagging
behindnode ‘A’,  by interrogating node ‘B’ only.   

From the BDR documentation on monitoring:

SELECT pg_xlog_location_diff(pg_current_xlog_insert_location(), flush_location) AS lag_bytes,
       pid,
       application_name
  FROM pg_stat_replication;

Because it is querying the pg_stat_replication table,  I will need to run this query on node ‘A’ to check the lag on
node‘B’, is that true?  I need to be able run a query on node ‘B’ to determine if it node ‘B’ is behind.  I am not sure
theabove query will work for that use case.