With the engine running and with the lights, heater and heated rear screen on, put one probe of your multimeter on the battery positive terminal and the other probe on the output terminal of the alternator and see what voltage you've got.

Anything under 0.3v I would say is ok but anything over would suggest you've got high resistance on that cable.

May as well check the earth side in the same way as well while you're at it.

if you've got high resistance, either clean up the connections or just make a new cable - job done!