What color is positive on a multimeter?
Every dignified multimeter uses red for positive and black for negative.

In addition to that, when you're measuring volts, you can just switch the inputs. It won't go wrong, you'll just get a negative value.

If you suspect it might be assembled wrong, just check it with a voltage source of which you know which pole is the positive and which the negative. Connect the positive to the red lead and the negative to the black lead. If the reading is positive, red is for positive. Otherwise, it is indeed assembled incorrectly, and you should use red for negative.