Genuine bumpers have clips that clip into the metal bumper iron. You can tell they are genuine by the corners of the black textured part. They will have a moulding line on each corner.
Pattern bumper are screwed into the metal iron bar and have no moulding lines.

You can change the plastic but you may aswel buy complete. You might be hard pushed to find a mint genuine bumper and vwheritage sell pattern ones that fit very well.