Modern Dynohubs from Shimano are so efficient that it is hard to tell if they are on or off. The pulse you feel when you spin the wheel by fingertip is a magnetic resistance followed by a magnetic acceleration to the next resting point, over the space of a few degrees rotation. When combined they almost cancel out.

Standlight headlights switch to dim standlight mode about 1/2 second after the wheel stops spinning. As soon as the wheel rotates it springs back to full power. Wheeling the bike it is on full power but flickers until you reach 3mph (German standards). If you run out of standlight at a long stop, you have to lift the front and spin the wheel a bit by hand but Ive never had to do this.

You can't repair a puncture by dynamo light, you need an auxilliary battery lamp. I carry a small LED watch battery backup.

A dynohub can be used to power the front only or a f/r combo.