AC, Speaker Polarity?

In general laymen's terms, loudspeakers are polarity marked in the "forward drive" direction for the voice coil.  When a positive electrical signal is placed on the positive terminal, the voice coil drives the speaker cone "forward."  The marking is for ease of setting up and building systems, as it is important to understand the electrical polarity of the voice coils so you can ensure your system is all "driving" in the same direction.  You want the speakers all moving "forward" together or else the sound pressure waves they create can cancel each other out.  But as long as all speakers in a system are in electrical phase, it really does not matter which terminal you use as positive as long as you use the same one on all of the speakers.  Was that clear at all?

Now in some crossover designs and some loudspeaker enclosure designs, the electrical phase becomes inverted by the system and you have to wire one or more speakers out of phase with the others, but that is another topic for another night.  Cheers.

With a Sharpie? 

Like I said, a + signal on the + terminal will ALWAYS drive the cone forward.  The fact that the polarity changes is why it's called alternating current, and without that you will have no sound.