In SharePoint 2013 I was missing a feature: Sign in as Different User. After a quick search, I realized that this function isn't available anymore.
BUT! There is always a solution for problems like this.
The first one would be, to run IE with another user account, which is recommended for productive systems.
The other solution would be to manipulate the "Welcome.ascx" file. 1. Go to your 15 hive 2. Go to \TEMPLATE\CONTROLTEMPLATES\Welcome.ascx 3. Add this code snippet before you find the "ID_RequestAccess"-id
<SharePoint:MenuItemTemplate runat="server" ID="ID_LoginAsDifferentUser" Text="<%$Resources:wss,personalactions_loginasdifferentuser%>" Description="<%$Resources:wss,personalactions_loginasdifferentuserdescription%>" MenuGroupId="100" Sequence="100" UseShortId="true" /> But remember: use this only on a development system.