At the request of fansite's admin, each of fansite has the right to have an item representing the site in game. Replicas are created if fansite is no longer active for some time (about 2 years). If you see the item representing currently active fansite, then you can be sure that it's original, rare item. The problem starts when you want to know if item representing ex-fansite is original.
The only differences are:
1) description - on fansite items you can see unique describtion + usually "Awarded by XYZ.com", on replicas you can read that it is a souvenir from Thais Museum. To read the description you have to stand close to the item.
2) replicas doesn't have additional animations - let me explain it. Fansite items has often 2 stages - static and animation. You can put small animation in a static version, like shine etc. You can also make only 1 stage with the only animation that item has. Replicas has only 1 "stage". So if basic version is static and after using the item you can see animation - it means that replica won't have it. Unfortunately if basic version of the item has the animation then replica will look exactly the same. In this case you can put replica far from the window and it will be unrecognizable.
3) sounds - not all of the fansite items have sounds, while replicas never have it.