Was it smooth and efficient or was it rough and awkward? Did they use their mirrors or did they only twist their body to look over their shoulder? Did they observe continually or was it a last second thought? Did they accelerate into an open gap without slowing anybody down or did they unnecessarily step on their brakes and cause people to slow?
All of this provides valuable information that you can use to learn about them, which you can then use in the moment and on down the road.