Desde http://converter.telerik.com/:
if (Page.User.Identity != null && Page.User.Identity is FormsIdentity) {
FormsIdentity ident = (FormsIdentity)Page.User.Identity;
FormsAuthenticationTicket ticket = ident.Ticket;
string AdminUserName = ticket.UserData;
if (!string.IsNullOrEmpty(AdminUserName)) {
//An Admin user is logged on as another user...
//The variable AdminUserName returns the Admin user's name
//To get the currently logged on user's name, use Page.User.Identity.Name
} else {
//The user logged on directly (the typical scenario)
}
}