I would recommend a meter... A test light coming on just means there "is" voltage. It may be 2 volts it may be 12 volts, there's no way to know. A meter doesn't lie
You can also pull one of the rubber covers off of the starter/generator. This page has some great pictures of what to look for
https://www.golfcarcatalog.com/golf-...rator-brushes/