There are many scenarios in which you may need a specific product ID from an uploaded product feed, e.g. when you want to use the Selected products (Json) logic.
To check the ID of the selected products, go to Settings in main menu bar on the left...
...and choose Product feed.
Then select the one you are interested in on the catalogue list and click the gear symbol. Select Show catalog to check its contents.
On the product feed information page, you will find the last update data, the number of products and a list of them. The list of products shows their thumbnail, title, price, as well as the ID.
Remember that the products you choose must be available – otherwise the frame in which they are to be shown will not be displayed at all!