Answer:
Carefully remove the 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 number will be stamped into the china or as a stamp with a barcode. This is the 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.
As reference, below are a few examples of how that tank information may appear in the tank.
