This question comes up a lot and unfortunately there is no easy answer. You can look through the autopilot screen or even test drive the car and enable the navigate on autopilot, which is a feature of FSD. Or you could see if lane change while on autopilot is enabled - also an FSD feature.
However, if the car is a 2017-early 2019 model it’s possible that the original owner purchased Enhanced Auto Pilot, which at the moment has the same features as FSD but does not entitle you to the HW3 hardware upgrade or any future FSD features like city driving or stop light recognition.
You will need them to call Tesla while you are there and ask Tesla to look it up by VIN to see if FSD was purchased...and hope Tesla gives you the correct answer.