First turn your multimeter to “Volts” mode usually signified by a V, next assuming your multimeter is normal connect your black lead to wire A and the red lead to wire B. If the output voltage is positive (+) then wire A is negative and wire B is positive, if the output voltage is negative then vice versa

Also 99.99% of the time is that black wires are usually negative but I know you wanted to be sure! And you guys are both right so don’t argue over this type of stuff lol, you just are kinda scared and you want to be sure, and he is probably a little frustrated, have a good evening!