Was quite simple in SharePoint 2010 but the "Log on as Different User" the has been removed in SP2013.
I Used it quite often.
This menu is not available on SP2013.
Hold down shift and right click Internet Explorer, choose Run as different user
Here is can supply the details of the user you want to log in as.
You can acces the site with the following url:
You can enter the details here.
Cant imagine why they would remove the link...