If your speaker cables don’t have any color marking, like red and black, here’s how to check polarity of your speakers for positive and negative wires.
First, remove the protection mesh by pulling on the sides.
Now, take a battery. Many recommend to use a 9 volts battery, but first, people rarely have one at hand, and second, 9 volts is too dangerous for lower power speakers. So, get an AA or AAA battery, which are much more readily available.
Then, briefly connect the battery to the speaker wires. If you see the center of the speaker moving towards you, that means that the wire you are touching the positive pole the battery with, is the red, plus wire. And wire versa, if the speaker center is moving away from you, that means that the wire you are touching the plus of the battery is the negative one. To mark the positive speaker wire, you may use correction fluid or nail polish. The easiest way to mark it though, is to just tie a small knot on the positive wire. The wire that makes the speaker go towards you, when you touch the + of the battery.
You may also use this simple test signal to determine if your speakers have polarity problems: