Unfortunately, there's no central way to look for this information that you can simply do from the Fortnite menu. You can choose from each storefront and run down the costs you have associated with Fortnite, or you could always check your bank statements for charges from the Epic Games Store. Most online banking apps let you sort purchases by source. You'd have to check each purchase. You should also receive emails when you make purchases on any platform, which can help you figure out how much of your money is going to Epic Games.