To do a basic test to establish that your dynamo is working :-

Lucas units

Disconnect the D & F (or F2) wires from it. Make a link wire up to connect the 2 terminals together. Between this common connection and the body of the dynamo connect a bulb (for a 6v dynamo use a 6v 21w bulb and for a 12v dynamo use a 12v 21 or 36w bulb). Start the engine and the bulb should light if the dynamo is good. Raise the revs and the bulb should burn brightly and may even blow. Don't do this for too long as the dynamo is running unregulated. 

Bosch and other Continental dynamos/Dynastarters

Disconnect the wires D+ and DF. Connect the DF terminal to earth (DE). Connect a 36w bulb across D+ and earth (DE) and run the engine. If the dynamo is good the bulb will light. Raise the revs and the bulb should burn brightly and may even blow. Don't do this for too long as the dynamo is running unregulated.