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One of the terminals should have about 3 ohms resistance between its self and the generator body. This will be the F.
The other should have not much between it and the generator body and it may vary a bit as the armature is rotated because there is two brushes between it and completing the circuit to earth. The F terminal resistance should not vary as it does not go through brushes.
This assumes it is a Lucas and has not been modified for say a Bosch regulator.
If you can see inside the geni the wire from the D terminal will go to one of the brushes and the F will go to the field coil. The other brush and other end of the field coil should go to earth.
The generators earthed through the body so a really good paint job will mean no earth. Rare that paint insulates that well with all the bolts and clamps and stuff but worth checking back to battery earth.