which wire is sending a + signal

OK, I had this new Pioneer HU installed a few days ago. My question is: is there a way to know or to check if the wires from new HU are conected corectly to each speakers polarity(- +) without disasembling everything. The sound is good, but I hear that even if the speaker wires polarity is inverted, its hard to tell since the sound its pretty closed to correct connection. Just wanna make sure.

If a speakers polarity is reversed then they have poor bass response. Listen for this from each speaker(use the fade and balance controls to check one speaker at a time).