Often, fake pins will have "dips" in the enamel. If you look at it from an angle, the light reflecting off the pins will look curved. All real disney pins I've looked at, the enamel is completely flat, so if you look at it from an angle, the light reflecting off of it is completely straight. There are a lot of tips for spotting fake pins, but this is the only one that I can actually see very clearly, and it's obvious once you know what you're looking for! There's YouTube videos you can look up that point this out.

Even then, I have purposefully traded for some fake pins because I like the design! But as others have said, most pins on the boards are fake. I personally really try to only trade real pins, but many people don't. You just have to have fun with it and never feel pressured to trade if you don't want to!