What is incumbent upon one to do to absolve oneself of responsibility for a car accident where a car's side mirror collided with another car, with uncertainty regarding the extent of the damage?

If your moving car hits a properly parked car, you are liable because you are the aggressor. You were obliged to stop and assess the damage. If the damage is significant, you must contact the car's owner and inform him to ascertain whether he will forgive you or demand compensation for the repair. However, if the damage is minor and insignificant, you are not liable for anything. If you are unable to contact the car's owner and are unsure about significant damage, there is no blame upon you, as the default principle is the exoneration of responsibility.