Always_Ask_Questions,
“Or get into habit of talking to the kid and have them on your mind, so you’ll notice that he is not answering.”
This won’t work for a small infant or a child who has fallen asleep in his carseat.
Regarding phone distraction – that’s a parallel issue. Good drivers presumably are not engrossed in their phones or sefarim while driving. Kids R”L were left in cars before the invention of the cell phone. The danger is going through a routine and forgetting or not realizing the child is back there. My stomach hurts just thinking about it. It can happen to anyone.