stress-testing MySQL slave with master's queries
I would collect queries from Master via slow log and then reply them on the slave using http://www.percona.com/doc/percona-toolkit/2.1/pt-log-player.html to do this you will need:


Enable full slow query logging. You need to be careful with the IO load if your server is really busy. You may want to do it during low time (say at night)
Collect good amount of queries to play with
Replay them using pt-log-player on slave using filter (you don't want to replay INSERTs/UPDATEs/DELETEs as it will ruin data consistency on slave and you'll have to recreate it.