Get yourself a cheap multimeter They are worth their weight in gold for a multitude of tasks and you can get one for about £5.
Set your new multimeter to test for voltage.
Put one multimeter probe on the positive terminal of the battery and the other on the negative terminal.
The meter will now show the level of your battery (in a 12v system this should be just over 12v, in a 6v system it should be just over 6v)
Start the engine.
As you increase the throttle, you should see a noticeable increase in the voltage shown on the multimeter (in a 12v system it should rise up to about 14v, in a 6v system it should rise to about 8v). If you do see this rise, your dynamo is charging fine. If there is no change in the multimeter reading with the engine running then something is wrong.
It's as simple as that!
Good luck
Simon
