jim is right, but also, if you have different types of speakers.
In multi-way speakers, designers will often flip a driver relative to others to achieve correct phase relationships. So if you are using mixed brands of speakers I could see this happening.
The absolute best way to tell is to use a battery. A standard AA will work, and is safe. Attach to each speaker, and test the direction the woofer moves in with the + of the battery attached to the + of the speaker.
You could probably also use a 9V if you need a little more oomph. :)
Best,
E