Free 3D Modeling Software - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 121

Figure 6.22 It is easier to confirm centered objects when looking from above.
The two objects are centered along one center line. If you rotate so you are looking
from slightly above and to the front of the top object, as seen in Figure 6.23 , you
will see that the objects are centered down the selected center line.