Whenever I have a sprite frame, and I open it in Preview app, I can't see the exact edges of the image because the background of an image is transparent. Therefore, I can't see if there is any padding around it. Is it possible to turn on image borders in Preview app?
You can toggle "Show Image Background" in "View" menu (⌥ alt+⌘ cmd+B). This will produce "chess" pattern instead of the clear color. Example (I've set white color around this rectangle to be transparent):