How can I know which cable should be "live" and which one should be "neutral", or does it not matter? (GU10 socket).
GU10 sockets are symmetrical:
So there is no need to pay attention to where you hook the live and neutral wires, because the lamp can be inserted either way. As the wiring is AC the main reason to wire it up one way or another is for safety considerations.
If you are wiring the switch too, put it on the live wire if possible, so that changing the lamp with the switch off is actually safe. The socket looks pretty safe to me anyway.