You can use the IS_MEMBER('role') t-sql function to detect what roles they have. I would probably wrap it in a stored procedure and return roles that I care about as a set of records (i.e., all the roles the user has that I care about).
Then you simply you use code like:
if (loginEntity.IsAdmin)
{
MenuItem adminMenu = new MenuItem();
adminMenu.Text = Resources.AdminMenuText;
mainMenu.MenuItems.Add(adminMenu);
item = new MenuItem();
item.Text = Resources.ManageUsers;
item.Click += UserAdminClick;
adminMenu.MenuItems.Add(item);
...
}
where you create your menus. (My loginEntity object here is calling a similar function and sets a bit variable for each role I care about.) The code generates a new toplevel menu (assuming that mainMenu is already defined), and then an "User Admin" item below it, all assuming the current users has an administrator role (say, 'securityadmin').