If this is a Lucas
dynamo, D and F should be connected together (and isolated from the rest of the wiring) for the test. It doesn't matter much what you use to connect them.
A voltmeter then connected between your D F and engine casing should show voltage rising over 20V as you rev up from idle.
A headlamp bulb connected between D F and engine casing should light brightly as you rev up.
If there's no dynamo output, try briefly connecting only F to battery live.
If It's a Miller dynamo with a built-in cutout, I know nothing about them.