Displaying the normals does not affect the model, but sometimes it can help you in your modeling to see just in which way your faces are pointing.
Press Ctrl+MMB, and use the mouse to zoom out, so that you can see the whole cube.
In the 3D View, press N to get the Properties Panel.
Scroll down in the Properties Panel till you get to the Mesh Display subpanel.
Go down to where it says Normals:.
There are two buttons like the edge select and face select buttons in the 3D View header. Click on the button with a cube and an orange rhomboid, the Face Select button to choose selecting the normals of the faces. Beside the Face Select button, you can adjust the displayed size of the normal, as shown in the following screenshot. The displayed normals are the blue lines. Set the Normal Size to
0.8. In the following image, I used the cube as it was just before you made the last extrusion, so that it displays the normals a little better.
Press the MMB, use the mouse...