The script must be executed as ‘postgres’ user.

#!/bin/bash

# $Id: check_slave_replication.sh 3421 2013-08-09 07:52:44Z jmorano $

STATE_OK=0
STATE_WARNING=1
STATE_CRITICAL=2
STATE_UNKNOWN=3
 
## Master (p_) and Slave (s_) DB Server Information	
export s_host=$1
export s_port=$2
export p_db=$3
export p_host=$4
export p_port=$5
 
export psql=/opt/postgresql/bin/psql
export bc=/usr/bin/bc
 
## Limits
export  critical_limit=83886080 # 5 * 16MB, size of 5 WAL files
export   warning_limit=16777216 # 16 MB, size of 1 WAL file
 
master_lag=$($psql -U postgres -h$p_host -p$p_port -A -t -c "SELECT pg_xlog_location_diff(pg_current_xlog_location(), '0/0') AS offset" $p_db)
slave_lag=$($psql -U postgres  -h$s_host -p$s_port -A -t -c "SELECT pg_xlog_location_diff(pg_last_xlog_receive_location(), '0/0') AS receive" $p_db)
replay_lag=$($psql -U postgres -h$s_host -p$s_port -A -t -c "SELECT pg_xlog_location_diff(pg_last_xlog_replay_location(), '0/0') AS replay" $p_db)
replay_timediff=$($psql -U postgres -h$s_host -p$s_port -A -t -c "SELECT NOW() - pg_last_xact_replay_timestamp() AS replication_delay" $p_db)
 
if [[ $master_lag -eq '' || $slave_lag -eq '' || $replay_lag -eq '' ]]; then
    echo "CRITICAL: Stream has no value to compare (is replication configured or connectivity problem?)"
    exit $STATE_CRITICAL
else
    if [[ $master_lag -eq $slave_lag && $master_lag -eq $replay_lag && $slave_lag -eq $replay_lag ]] ; then
        echo "OK: Stream: MASTER:$master_lag Slave:$slave_lag Replay:$replay_lag"
        exit $STATE_OK
    else
        if [[ $master_lag -eq $slave_lag ]] ; then
            if [[ $master_lag -ne $replay_lag ]] ; then
                if [ $(bc <<< $master_lag-$replay_lag) -lt $warning_limit ]; then
                    echo "OK: Stream: MASTER:$master_lag Replay:$replay_lag :: REPLAY BEHIND"
                    exit $STATE_OK
                else
                    echo "WARNING: Stream: MASTER:$master_lag Replay:$replay_lag :: REPLAY $(bc <<< $master_lag-$replay_lag)bytes BEHIND (${replay_timediff}seconds)"
                    exit $STATE_WARNING
                fi
            fi
        else
            if [ $(bc <<< $master_lag-$slave_lag) -gt $critical_limit ]; then
                echo "CRITICAL: Stream: MASTER:$master_lag Slave:$slave_lag :: STREAM BEYOND CRITICAL LIMIT ($(bc <<< $master_lag-$slave_lag)bytes)"
                exit $STATE_CRITICAL
            else
                if [ $(bc <<< $master_lag-$slave_lag) -lt $warning_limit ]; then
                    echo "OK: Stream: MASTER:$master_lag Slave:$slave_lag Replay:$replay_lag :: STREAM BEHIND"
                    exit $STATE_OK
                else
                    echo "WARNING: Stream: MASTER:$master_lag Slave:$slave_lag :: STREAM BEYOND WARNING LIMIT ($(bc <<< $master_lag-$replay_lag)bytes)"
                    exit $STATE_WARNING
                fi
            fi
        fi
        echo "UNKNOWN: Stream: MASTER: $master_lag Slave: $slave_lag Replay: $replay_lag"
        exit $STATE_UNKNOWN
    fi
fi

Possible outputs:

$ bash check_slave_replication.sh 192.168.0.1 5432 live 192.168.0.2 5432
WARNING: Stream: MASTER:1907958306184 Replay:1907878056888 :: REPLAY 80249296bytes BEHIND (00:03:14.056747seconds)
$ bash check_slave_replication.sh 192.168.0.1 5432 live 192.168.0.2 5432
OK: Stream: MASTER:2055690128376 Slave:2055690143144 Replay:2055690193744 :: STREAM BEHIND
$ bash check_slave_replication.sh 192.168.0.1 5432 live 192.168.0.2 5432
OK: Stream: MASTER:2055690497120 Replay:2055690497328 :: REPLAY BEHIND
$ bash check_slave_replication.sh 192.168.0.1 5432 live 192.168.0.2 5432
OK: Stream: MASTER:2055691704672 Slave:2055691704672 Replay:2055691704672