I'll probably get in trouble for recommending a site but pinpics.com, has a gallery with pictures of all pins. With a bit of searching you can find the pin you have (using the year of the pin and a description like a character or attraction) - then you can compare your pin with a picture. The overall finish on a scrapper is not as good as a real pin, with thicker lines or the colours being off. Certain pins are copied more than others and it usually has a warning underneath with a little hint to help you determine if you have a scrapper or not.
Hope this helps! Once you get good at spotting scrappers you can avoid trading for them in the parks too