How can I hear if my sub is phased correctly?

I can hear it on normal speakers, but this is my first sub ever..

I have an M&K sub hooked up to a pioneer elite 37. My cable is 1 plug into the receiver and 2 into the sub.. and I cant see which should go where.

I can change +/- on the sub.

Sorry if this is a dumb question - first sub :-)

Usually, you can switch the leads and which ever position plays louder is in phase. You can also use a test disc like one Stereophile's test discs or Avia.