FeedForAll has a feed validation function built in. It will automatically attempt to validate any feed that you open, and will again attempt to validate you feed when you save or publish it. If there are no validation errors, warnings, or automatic repairs, the validation log screen won't even show up. Otherwise, you will be presented with the Validation Log screen, showing the problems with the feed. In the Miscellaneous section of Preferences you can change when the Validation Log screen will appear.
To manually start the validation function, click the 'Validate Feed' button in the toolbar, or choose 'Validate' from the 'Tools' menu. When the Validation Log is showing you can click on any of the errors, warnings or repairs, and you will be brought to the relevant field on FeedForAll's main screen.