On the top left hand side of the darkroom, the navigation panel displays a full preview of the current image with a rectangle showing the currently visible zoom area in the central panel. Drag the rectangle around to pan the zoomed-in view. The current zoom scale is displayed to the right of the preview image. Click on that figure for a quick access to some common zoom levels.
The navigation panel can be toggled on/off with a keyboard shortcut (Ctrl+Shift+N by default).