Originally Posted by
BNSF
Obviously, the safest technique would be to peer into each car window and see if the driver's hand is on the door latch, or not. If they're playing with their phones, or applying makeup (to name a couple examples), they're not likely to pop that door open any time soon. Of course, I'm not sure if any mere mortals such as the rest of us are quick enough to see the driver's hand, not at 20 per.
Well, that and the door latch is usually a bit below the window line. I still think it would be crazy not to assume that it's likely that a head on that side of a parked car means an extremely high probability of someone opening the door.