You can use the Force.com IDE to download your org's metadata (triggers, classes, workflow rules, etc.) and then search through the metadata for the name of the field in question. The search results will contain all references to that field, making it easy to identify which code is setting it.
You can enable debug logs on a particular user and save a record.
See the debug logs it will give detailed information about the action behind the save record which updates the field.
http://www.salesforcegeneral.com/salesforcecom-debug-logs/
Regards,
Ashish
Force.com IDE Installation (http://wiki.developerforce.com/page/Force.com_IDE_Installation)