Charcoal or even colored sidewalk chalk. Not sure where your growth ring is? Give it a rubbing, wipe with a rag and the charcoal or chalk sticks in the crumbly early wood, makes I really pop!
Sunrise and sunset has always been my best time to chase rings, when that light is angled low. I point the stave in line with the sun and go to town. I have a desk lamp that has an adjustable arm and I can mimic that sunrise/set condition. It's one of those smokin' hot halogen lamps, so it really throws some powerful light, too. If I shut off all the other lights in the garage that helps, too.