How to determine speaker polarity?

Okay, I was probably building subwoofer boxes, and installing stereos while most of you were in diapers, but I've never done something like this. I am replacing the factory speakers in my 2004 F-150.  The question is...how do you tell which wire is the positive wire, and which one is negative? If I look at my new Infinity's from the bottom, the positive is on the left side, and the negative is on the right side. Is this universal to all speakers?

No, it's not universal.  Usually Fords use a striped wire and a solid wire for factory speaker connections.  The striped wire is usually negative.  As long as you are consistent  on all speakers you will be fine.

I was out of MY diapers in 1952, by the way.  

I'm out of diaper, but give me another 35 years and I'll be in them again

BTW: You can do a 9 volt battery test on the speaker's to figure out the phase of speakers. If the cone extrudes, then where ever you have the (+) of the battery this will be your (+) lead and if the cone goes in, then it's the opposite.

Both wires are striped!!! That's my problem. Yikes.

Thanks for the advice.

Then you either need the vehicle wiring diagram, or use the battery pop test like Jeff suggested.  Or you can purchase Ford speaker wire adapters if your truck uses the standard 2-position Molex connectors.  Bottom line is it does not matter as long as both speakers are wired the same polarity.  Do the pop test from the head unit or amplifier.

Dag nabbit!! I bought a 9 volt battery, and I got news for you boys, touching the positive of the battery to either side of the speaker does absolutely nothing for the speaker cone.

B.T.W. I wired them up wrong I guess. I assumed the clip side was the negative. Guess I'll have to tear into it again tomorrow, and fix it.