how do you know if the radio is a Harmon Karden?

Check with dealer, they should be able to tell you the original radio option.

since you have a wagon you should have a subwoofer in the back if its the AS3 (or harmon kardon) or you can pull the head unit and look on it to see if it has the designation AS3 on the sticker.

My speaks say 'Harman/Kardon' on them and it's a 9speaker setup (on the Aero). I'm not sure if the head unit factors into it.

that will do it as well. the head unit on them is a big difference. there are three different stereo levels (AS1, AS2, AS3/HK). since you have a sedan your car will have the 9 speaker system with 2 speakers on the rear deck. whereas in the wagons they use just one subwoofer located in the back, which i dont think says HK on it anywhere.

I say turn the bass and treble up approx. 3/4 max. Then pop in a Led Zeppelin CD (preferably "IV") and recline the leather seat back and set volume to MAX. If you have the Harmon Karden system there will be no mistaking its clear, crisp sound along with a thumping you can feel with every one of Jon Bonham's bass drum kicks!! The other systems you will hear crackling noises and jibberish.....