MySQL has a great facility called the slow query log. It allows you to filter out the most expensive requests against your database. You may not have enabled it, however, and perhaps don’t want to restart your database. In cases like these, a quicker way to capture queries is through tcpdumps of the port MySQL runs on.
Maatkit’s mk-query-digest has a tcpdump mode which can magically analyze this traffic for you, and produce a spectacular report of your top resource intensive queries. In this Database Journal article we give it a once over.
Database Journal – Troubleshooting MySQL with Maatkit & Tcpdumps