Thanks to Artem Russakovskii for this nice explanation of what Seconds_Behind_Master means
In order to see if a slave is lagging, execute the 'show slave status' command and look for the Seconds_Behind_Master value. The way this value is calculated can be slightly ambiguous and unclear, so I'll explain. It is simply a difference between the 2 timestamps the time of the last received (and queued up in the relay log) query that already executed on the master and the time of the currently executing query on the slave.
As he says in the post, this value is not a precise indication of the replication delay.