If one speaker is out of phase, then it will cancel with the sound from other speakers. This is most noticable with bass frequencies which will largely disappear.
What's written on the cable itself doesn't matter, just that positive on the amp ends up as positive on the speaker. If you can't hear anything wrong, then you've nothing to worry about.
Do you have any DVDs with the THX Optimiser extras? They're pretty good for basic checks.