You can spot someone's feet under the width of a bus when they're on the opposite side of the bus and you're sitting in a vehicle at a much higher position on the opposite side that the bus is on? That's physically impossible.
Or you look for reflections in the cars parked around it. This is what I was taught as “defensive“ driving.
Googling this turned up a presentation from Waymo saying they do exactly this: https://www.reddit.com/r/waymo/comments/1kyapix/waymo_detect...