Channels: Listed products validation and feedback
While the 'listed products' will display some errors in regard to offer and content not being published on a channel, for more details you can use the 'Validation and feedback' tab in the Listed product section.
If a product is not exported (or in some specific cases rejected), the reason or reasons (usually missing attributes) are displayed in a grid. If you supply the missing attributes, in the next export the product will most likely be gone from the validation and feedback tab (unless it's an offer error, like your account not being permitted to sell a specific brand or product). This validation only checks if an actual value is entered - it does not full validation like checking if a number field is filled with an actual int.