Finding the culprit of 100% CPU usage?

Linode alerts keeps sending me emails every 2 hours:

Your Linode, xxxx, has exceeded the notification threshold (90) for CPU Usage by averaging 97.5% for the last 2 hours.

I used 'ps aux' to find it's one of the apache process:

www-data 12238 100.3 3.8 45956 19048 ? S 01:38 0:00 /usr/sbin/apache2 -k start

And there are also others of them, persisting there. I have no idea how to find which PHP script is the culprit for this. Anybody cares to enlighten me on this? Thanks! :roll: