The Backorder Report will show you all orders currently on backorder broken down by SKU.
It will also include the date the order was placed, product information, qty ordered and what might have already shipped if other items were ordered that are not on backorder.
A caution on allowing backorders
To ensure shoppers aren't waiting long to receive their order, we generally caution allowing backorders if items aren't going to be checked into inventory relatively quickly. With that said, we do have the ability to allow backorders. If an item is on backorder, it will list that on the product listing and also include this note on the confirmation email the client receives.
Please view this article for how to access reporting.