Simply put - the pieces look different. Edges, shapes, and curves set the various makes apart. After you look at various images of the different kits over and over, you'll be able to pick them apart too. None of us got lay of the land overnight, it takes time and study just like anything else
As for where the parts came from? Some people buy armor, then cast it to make copies. Some people sculpt armor from scratch and make their own pieces. Some people cast other people's pieces and then tweak or add their own parts to the mix.