Memory Usage of Each Application Pool

• Go to IIS Manager
• Click on the name of your server in the Connections pane
• Under the IIS section in the Features View, double-click the Worker Processes icon

Here you can see both the private bytes and virtual bytes usage of each running application pool.

More information can be found here: http://technet.microsoft.com/en-us/library/cc771273(v=ws.10).aspx