The injector signal consists of pulses, as seen in
this thread, so it would be hard to read with a multimeter, since the pulses happen so quickly.
I assume the VSS signal is similar, and the pulses occur more frequently the faster the vehicle is going, but I'm not positive. So again, it would be difficult to read on a multimeter.
If you see the voltage varying on one of the wires, then it's probably the VSS or injector signal, since they're the only ones that should change. Both the +12V and GND signals should remain constant.