Audio Speaker Polarity -- How do you check?

[Bradster]

I have a 97 base with an aftermarket Sony Xplod cd player. In the door were some old speakers that did not work and the wires were not marked as to positive and negative. Aftermarket, not original, I assume. I bought some Sony Xplod matching speakers for the door. Once installed, it does not seem to matter which way the wires are connected. In the past, if you wired them wrong, it would be obvious. In this case, the speakers sound good either way.

There must be a way to check the polarity without pulling the cd unit out of the dash. The wires in the car door are probably aftermarket as well. The passenger side has a red wire and a plain wire. The driver side has a plain wire and a blue wire. Any ideas? I am getting a buzz out of my right dash speaker, but that is my next job. The speaker buzzes no matter the connections on the door speakers.

Thoughts?

Thanks.

Bradster