Instructions
Identify the action figure's part that you think is a reproduction. Testing every piece can be time-consuming, so try to narrow it down.
Check the part's color. If the color of the piece is noticeably different than that of the rest of the toy, it's likely a reproduction. Try scratching the plastic with a sharp instrument. If color chips off and you can see that the plastic underneath is a different color, that indicates that the plastic part has been painted and, therefore, is a reproduction part; the color of all original Star Wars figures comes from the color of the plastic itself, not from paint.
Bend the plastic part, particularly if it's an accessory, such as a weapon. Many reproduction pieces are accessories. Reproduction accessories typically are made of rather inflexible plastic that will break if bent too much, although some reproduction parts may be overly pliable.
Examine the finish. Reproduction pieces have a rougher texture than original parts and are less polished than authentic pieces.
Drop the piece in question onto a hard surface and listen to the sound it makes. Reproduction pieces often are made of a different type of plastic than original parts and tend to make a "lighter" sound when dropped than an authentic part. This is a tricky test to perform since it takes a good ear to hear the sound difference. You can practice discerning the difference by dropping a piece that you know is authentic and then dropping the part in question and comparing the two different sounds.