Server 2008
Start Task Manager
Find w3wp.exe process (description IIS Worker Process)
Check User Name column to find who you’re IIS process is running as.
In the IIS GUI you can configure your application pool to run as a specific user:
Application Pool
default
Advanced Settings
Identity
Here’s the info from Microsoft on setting up Application Pool Identites:
http://learn.iis.net/page.aspx/624/application-pool-identities/