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Preview

On the top right of the App Builder, you'll see a "Preview" button. Preview shows you what your published app will look like and helps you to de-bug your app.
On the far right of the preview screen, you'll find a bug icon. Click on the icon to open up a panel. The Log tab will display all interactions that have happened -- including the event's Event Payload and the action's Transformed Parameter -- so you can check if your events are sending the correct values.