First thing to check is the connections for the battery and the reg/rect.
Generally there are three wires comming from the stator (altenator) If you measure the voltage between any pair of the three wires you should get around 70 V AC at 5K RPM. If you get a very low reading on one pair. the stator is history needs to be replaced. If the stator checks ok, wiring is Ok You need a new reg/rect
A basic charging system check can be done with a voltmeter. Put the meter across the battery terminal and check the voltage at idle. Then rev the engine up around 4000 rpm or so. The voltage should rise up very close to 14v or so. Also the headlight should brighten up some. If it doesn't thats one indication that the charging system may not be working. Other than that, the above post has it covered..