If it was a florescent light, even a simple vacuum tube, it would flash with nearby activity. You could see it from the fields and know that it was time to hotfoot it inside. However, back in the day, they could have been depending on seeing a static discharge like St. Elmo’s fire around the glass just prior to a strike–time to hit the dirt! (A crouching kneel actually.)
eta: Hmm. Yup, they’re suppose to break if there’s an actual lightning strike, and since the lightning rod goes through the center, there’s isn’t going to be a coronal discharge around or in it. Tsk, pity! Attach a florescent tube to the end of your lightning rod, it’ll be fun!
https://www.peachridgeglass.com/2012/04/lightning-rod-balls/
