I assume the DRV2 is connected up for 6V operation. I'm not sure whether the wire colours are different in your case but, in the DRV2 instructions I can find, the brown and white wires must be connected together for 6V. When you say purple wire, I assume this is the same as the brown wire in my set of instructions which corresponds to the old A Terminal?
As with most regulators, there is relatively little that can be tested and it either works or it doesn't. However, for the DRV2, I would measure the voltage generated between the earth and the yellow (D) and the same for the green wire (F) when the engine is revved. When the voltage reaches at least 6.3V or so on each, the voltage should appear on the brown (or is it purple) wire which I would disconnect from the battery during the test.
Providing the DRV2 is wired correctly and the battery is in good condition, the regulator should work as intended unless there is a fault with the dynamo which does not appear to be the case. The one thing that can cause the regulator not to work is if it is not properly earthed to the dynamo and it would therefore worth checking the resistance between the DRV2 earth wire and the dynamo earth.
