Model numbers can be found on the product labels of every box. The box your product came in should have one or both of the two types of labels shown below. The areas highlighted in red below show the Model Number.
Carefully remove your tank lid, and place it gently on the floor. You'll notice a four-digit number located on the back of the tank, near the water level mark. This is your tank model number. If the number starts with a two, it is a one-piece toilet. If the number starts with a four, it is a two-piece toilet.