If you employ dark fly lines, a dab of white or fluorescent red paint near the line-leader knot helps detect strikes. Experience at bluegill nymphing will do more for you than all the paint marks, cloth strips and white lines combined. That sixth sense of knowing when to strike does exist. In time, it will grow so you'll respond to strikes when the light is so dim a kind of trained hunch guides you. Nymphs are so deadly for bluegills that I believe any fish that refuses such a fly is either sick, gorged from feeding, or is in a dormant state.
Nymphs take bluegills wherever the panfish are found.
Work nymphs around weed beds, flooded timber, brush piles, off deep sand bars, and near undercut banks in streams and rivers. It's usually a good idea to begin fishing close and shallow. Then slowly lengthen casts and work a nymph deeper until the fly is finally scraping bottom. At any depth, nymphs are great bluegill catchers. In fact, one of my favorite methods is to allow a fly to sink all the way to bottom, then retrieve it slowly with a slight jiggling action imparted from the rod tip.