The Face and Gloves
The first area to look at is the face. The authentic Unmasked Jason Voorhees has a very complex color and shading mixture. A fake is not going to come close to the same coloring and in fact will look flat and lacking in detail. The authentic will actually have a very dirty finish to it.
The next item to look for is going to be the gloves. Just like with the face, the gloves will have a flat leather black color. The fake will have jet black and glossy gloves.
Bottom of the Feet
The tale-tell sign for most fake Funko Pop! is going to be the bottom of the feet. Check the bottom of the Pop! if you see A202 this is bad and you have a fake. If you see A285 you have an authentic.
When looking at the box there are a number of key indicators that Jason Voorhees is a fake. The 3most obvious are the box art quality, the sticker, and the typography.