Getting a closer look at the canvas with the Zoom tool
Use the Zoom tool to get a nearer or farther view of the canvas. 
Click anywhere on the canvas to center on that point and double the zoom percentage. Option-click anywhere on the canvas to center on that point and halve the zoom percentage.
Click and drag a box across the canvas to specify the rectangle you would like to zoom to; the shading shows the area that will be visible in the window when you let go. If you hold Option and then drag, the rectangle is centered on the spot where you start dragging. While dragging, you can hold Shift to move the rectangle around instead of resizing it.
Shift-click the zoom tool in the tool palette to return to 100% zoom, "actual size".
You can always zoom to a specific percentage with the zoom menu in the lower-right of the canvas window.
If you have a MacBook with multi-touch capability, you can use it to change the zoom level. First, deselect all objects. Then, either pinch open to zoom in, or pinch close to zoom out.