How do I know if my subowoofer phase should be normal or reversed?

 

Personal preference.

I almost always reverse mine.

 

you should be able to hear the difference.

 

You mean subwoofer POLARITY, meaning reversing the wire at the terminals. Phase is a misnomer for that. Lots of manufacturers label a polarity switch as phase reversal too so it gets kind of confusing.

Like Titand19 said - you should be able to hear it. If you have two subs together and you flip the polarity of one - it'll sound like you almost unhooked them from inside the car (cancellation). You have the same effect with the mids-to sub but not quite as noticable. You should really play a song that has a nice rock beat (kick and snare). Now listen to it. Does the kick hit at the right time? Or does it sound off? For the general idea, I like to use the newer Creed CD, track #4 (I forget the song). It has an odd beat that makes it easy to tell when it's off. From there I check with some test equip, but that should get you close enough.

Now back to the phase thing. Your sub is probably at a right angle to your front stage unless you have HLCDs or kick panels. This puts the sub 90 degrees out of phase (basically it's a time thing, the one wave is shifted from the other). However, the extra distance the sub has to travel would pretty much make up for that (again a time thing) - but one polatity setting should sound the best, either normal or reversed (to line up the waves).