Shell Script / Tool to check the MySQL Replication Status?
If you don't have a monitoring framework in place, I would highly recommend looking into Nagios. There are others, but for the purpose of this question, Percona has many MySQL monitoring tools available. Two of them specifically monitors slave lag and slave failures. From their blog post:


  The pmp-check-mysql-replication-delay plugin checks for excessive delay, and pmp-check-mysql-replication-running checks that replication is functioning correctly.