Take the rear bumper off, it couldn't be easier, just 7 10mm bolts and two clips under the wheel arches.
Behind it you will either see some square black impact absorber things that should have absorbed some impact (!), these may be damaged if they've done they're job and if you want them to absord more impacts should be replaced i suppose.
If the car is a high range model or gti etc it might have a beam behind the bumber instead of the impact thingies, this beam is for impact absorption aswell and may be bent or buckled and could also be replaced if you thought it necessary.
Be carefull to remove the clips from the bits behind the wheel arches before refitting the bumper as it's impossible to refit it properly with them attached, they should be reclipped on when the bumper is in position.