In dealing with speaker wires, the positive speaker wire will be silver in color. This is only visible if the insulation is clear or see-through so that color can be distinguished. The negative wire will usually be copper in color for this same instance.
However, there are ways to tell charge type even without being able to see the wire's color. Sometimes, when professionally installed, the positive wire will have a stripe of any color, whereas the negative one will not. The simplest and most accurate way to determine this, though, is by checking the connection point at the speaker, which almost always has some denotation of positive or negative.