lsof is a helpful tool for looking at what files a process currently has open, however sometimes a process may only access a file for a second and lsof may miss the moment.

For OSX we also have Instruments. This is included with XCode and is pretty straight forward to use:

• Open Instruments
• Select File Activity
• Select the process
• Hit Record
• Perform your action
• Stop Recording

You can also save the log for later analysis.