Open Grid & Layout and enable Snap and Sizes, or simply use the shortcuts: S (for Snap) and X (for Sizes). The distance between the two aligned widgets will now be shown.
This feature is extremely helpful if you need to use multiple widgets with a constant distance between each instance (say, to create an image gallery). To do so, copy a widget with Option/Alt + Drag, set a distance between the original widget and its first copy, then repeat as many times as you need. The Sizes function will keep the distance between all copies equal.