Is there a way to test a voltage regulator to see if it is working properly?

 

On what kind of bike?

In general: measure AC voltage btw each leg of the stator. The voltage varies depending on which stator you have, but should be the same btw each of the legs and should increase as rpm increases. You can also check DC voltage at the battery. It should be ~12.7ish with the engine off, 13.5ish at idle and 14.4ish at high rpm.