To make sure you center it correctly, you'll need to remove the dust cap. Then shim the voice coil and measure the gap on all sides to make sure its centered.
If I were to "free air break in" a sub... I would play a test tone at the woofer's resonant frequency on my home receiver. I hook up my laptop that has a tone generator to the receiver and increase volume until the sub is moving around half of it's xmax.