SharePoint used to have a menu option called “Sign in as Different User” in the top-right corner of every page. It was a handy tool for developers and IT professionals in SharePoint 2007 and 2010, which allowed them to login with another account to test their solutions or customizations. Microsoft removed the option in SharePoint 2013, but you can still do it with workarounds or a one-click bookmarklet.
Navigate to the Close Connection Page
http://<site URL>/_layouts/closeConnection.aspx?loginasanotheruser=true
Reference: http://blog.credera.com/technology-insights/microsoft-solutions/how-to-sign-in-as-a-different-user-in-sharepoint-2013/
