show slave status \G
Important to look out for: 
Slave_IO_Running: Yes
Slave_SQL_Running: No
The following means that the master and slave are communicating. The master is sending over the IO commands and the slave is receiving them 
Slave_IO_Running: Yes
The Slave is NOT running the IO commands sent from the Master 
Slave_SQL_Running: No

Slave Behind Master?

mysql -e 'show slave status\G'|grep -i behind

Slave SQL/IO Running?

mysql -e 'SHOW SLAVE STATUS\G;' | grep -i slave*running