I don't know what decoder you're using but on Digitrax Decoders there is a real esy way to check this functionality: Make sure that the analog mode is enabled (in CV29) then just apply a DC voltage to the wheels. In N scale, 9V battery is perfect for this. It'll make the decoder "think" that it is on a DC layout and it will turn on the motor. This bypasses any speed curves or other customizations.
This verifies that the decoder is properly wired and also tells you that the decoder's motor output power transistors are ok. It also tells you that the microcontroller is somewhat sane as it has to turn those transistors on.
Measuring orange/gray voltage is somewhat tricky as it isn't either DC or AC. It is a pulsed voltage which can fool multimeters. But if there is voltage, think about it, the motor should be spinning (or making noise). Unless it is binding mechanically.
In that case, disconnect orange/gray wires and apply the 9V battery directly to the motor. If it doesn't run then you have a mechanical problem.
Peteski