To add to wazer's answer, when you're looking at pictures of the used car, you can tell whether or not it has ACC by looking at the steering wheel switch.
Fusions with ACC have a rocker switch on the lower left with two arrows:
Fusions without ACC have this switch instead with the words RES and CNCL in place of the two arrows
It's often the case that the Autotrader or Truecar listing incorrectly reports the options, saying that the car has ACC when it doesn't, or not specifying the existence of ACC when the car most definitely has it. The car I bought also failed to mention that the ACC exists on its TrueCar listing.
Thanks Wazer and 4cylinder ☺️