Rich Domain Model Implementation

I recently started reading about rich domain model instead of anemic models. All the projects I worked on before, we followed service pattern. In my new new project I'm trying to implement rich domain model. One of the issues I'm running into is trying to decide where the behavior goes in (in which class). Consider this example -

public class Order
{

   int OrderID;
   string OrderName;

   List<Items> OrderItems;
}

public class Item
{
   int OrderID;
   int ItemID;
   string ItemName;

}

So in this example, I have the AddItem method in Item class. Before I add an Item to an order, I need to make sure a valid order id is passed in. So I do that validation in AddItem method. Am I on the right track with this? Or do I need create validation in Order class that tells if the OrderID is valid?