If you go to your Orders page found under Account, you will see different statuses under the STATUS column of all your orders list, such as To Be Shipped, On Hold, Shipped, Partially Shipped, etc.
When your order has been shipped, you will see this status there.
Or you can click on the tab Shipped, To be Shipped or Partially Shipped on top of your Orders list and check if the order is there.
And that's how you know if the Supplier has shipped the order.