The normals of faces and vertices are useful to check for problems in lighting. It can also be useful to diagnose why a mesh is non-manifold.
First, make sure the Properties side bar is visible. This side bar can be toggled by choosing View > Properties. It can also be toggled by pressing the
Next, in the Properties side bar, go to Mesh Display > Normals. Select
Face to view face normals and
Vertex for vertex normals.
You can also adjust the length of the displayed normals by changing the
Normal Size value.
Tried with: Blender 2.62 and Ubuntu 12.04 LTS