When a person is aiming for his ideal weight, the most dissapointing and significant tool is the well beloved bathroom scale. You step onto it and watch the numbers on the display go up like crazy. The scale won't lie to you, as long as it isn't wrongly adjusted. So, to check if the diet did deliver what was promised you can use a bathroom scale.
They are made for the purpose of weighing persons mainly, other activities seem to be out of place. The technical development has changed these scales from bulky analog style to slim and fast electronical processors with LCD-display and high precision.