ASP.NET Identity 2.0 check if current user is in role IsInRole
Assuming you are in ASP.NET, it's pretty simple:
if (!Roles.IsUserInRole(User.Identity.Name, "Administrators")){ return "You are not authorized to access this page.";)
(from http://msdn.microsoft.com/en-us/library/4z6b5d42%28v=vs.110%29.aspx)