I use two or more rear lights to minimize the risk of running ninja. My helmet always has front and rear lights that can be switched easily and visibly from steady to flashing.

Even my short hop local errand bike has two rear read lights -- a Planet Bike Rack Blinky 5, and a wheel powered magneto blinker. Usually I also wear a helmet with front and rear lights. My immediate neighborhood is the most dangerous part of any ride so I want to be seen. For the first mile in any direction it's all transient apartment complexes. Hardly anyone lives in this neighborhood longer than a year, they have no investment in any sort of "community" so most drivers nearby are reckless and indifferent. But as soon as I get a mile away we're into older stable residential homes and drivers are much more courteous and considerate.

When in doubt, I pull over to check.

With the Cygolite Hotshot I can check my mirror and see the light reflected off objects behind me, or even off the pavement. Unless I'm running steady-dim, which I use only for nighttime group rides or on the MUP. As soon as I'm in regular traffic I pull over for a moment to change modes.

With the helmet light, I can see the light and mode reflected off my shoulder or hand in my helmet mirror or handlebar mirror. Since I run a Blackburn 2'Fer with red/white-steady/flashing, my main concern is not inadvertently running it in white mode on the back of the helmet. It has a single button to cycle through all modes and a couple of times I've inadvertently switched to white steady or flashing when I was wearing gloves while operating the light without looking.

My lower power steady/flashers aren't readily visible through the mirror so I either trust that they're on, use a backup light, or stop to check. That includes my old style rear LED/reflector lights and flashing LED saddle light.