If the wires are hooked up in the main panel, then all you'll need is a multimeter/voltmeter.
- Carefully position the wires so that there's no chance of them touching each other, or anything else.
- Turn on the breaker in the main panel.
- Carefully measure the voltage between each set of wires.
- Turn the breaker back off.
When measuring voltage between the two ungrounded (hot) conductors, you'll measure about 240 volts. When measuring between either ungrounded (hot) conductor and the grounded (neutral), you'll measure about 120 volts.
Alternatively, if you're not comfortable working with live wires (and really, who is).
- Make sure the breaker in the main panel is OFF.
- Using a multimeter set to measure resistance or continuity, check for resistance/continuity between the grounding conductor and the other wires.
You'll measure infinite resistance (or open) between the grounding and ungrounded (hot) conductors, but you'll measure a small resistance (or closed) when measuring between the grounding and grounded (neutral) conductor.
Examine the wires carefully. If they are part of a cable assembly, one of the wires may have stripes on the insulation or some other markings. If these are individual wires, there's likely no markings unless the installer marked them with paint or tape.
Once you find the grounded (neutral), mark it with white paint or white electrical tape.