You can check your pins on PinPics. The scrappers usually have visible defects, like weird coloring (I had a pin where Snow White looked faintly green). Counterfeits will have inaccurate back stamps or other much more subtle variations. Other things to stay away from are scratched finishes or metal that feels much lighter (almost like plastic).
We just came home from WDW and we have several pins that I am pretty sure are fake. I haven't had time to check them, though. On the "boards" section of PinPics there is a comparison forum where people post real and fake pins side by side with the differences numbered so you can see what's wrong with the fakes. Of course these differences are very hard to see with the naked eye - they are much more apparent when you have a real one right beside the fake.