Try my replicaiton check, developed to run under Nagios:
http://opensource.fotango.com/svn/trunk/systems/nagios_plugins/check_replication.pl
Configure the replication user on your master to be on your slave as well, and give it the right permissions (as in the --help). Then start the script with --slave --slave-pass and it connects to the slave (use --slave-port if required). It reads the status, determines the position, and also the address and port of the master. It then connects to the master server with the same credentials, does a SHOW MASTER STATUS, and compares the differences.
Output is like:
OK: 0.000 diff 5 secs, 192.168.0.1:3306 (4.1.7-standard-log) -> 192.168.0.2:3306 (4.1.7-standard-log)
A newer option I have added is for the script to look at the databases that are being replicated, and then do a SHOW TABLE STATUS on one of these (randomly selected) and do the same on the master, and then check they have a similar number of rows. This is a rough check that not only is replication working, but the table sizes are approximately the same. NOTE: InnoDB does (wildly random) approximations of rows in SHOW TABLE STATUS, so these table types are ignored.
Hope this helps.