If am not mistaken, in order to find out in which nodes your rows are stored, you would need to run:
nodetool getendpoints keyspace columnfamily 2013-07-30:1
Use your (composite partition/) row key instead of your column key.
If you are using the SimpleStrategy for token/replica calculation, this will happen internally:
You compute the MD5 hash of the key. Create sorted list of tokens assigned to the nodes in the ring. Find the first token greater than the hash. This is the first node. Next in the list is the replica, which depends on the RF.
(found this on the cassandra mailing list http://cassandra-user-incubator-apache-org.3065146.n2.nabble.com/How-to-find-what-node-a-key-is-on-td6202253.html)
(Updated my answer according the comment.)