If the thomson model of the atom had been correct, rutherford would have observed:

Before Rutherford's Planetary Model, the widely accepted model was J.J Thomson's Plum-Pudding Model. In Thomson's model, an atom is a positive sphere containing protons initially. Then, the electrons are embedded in the atom to neutralize the positive charges making the atom neutral. Rutherford experimented Thomson's findings. In his gold foil experiment, he would bombard the atom with a stream of alpha particles. If Thomson was correct, the light should absorb most of the light, may diffract a little, but not reflect back to the source. However, the opposite happened. Then, Rutherford disproved Thomson's model and proposed that there is a nucleus at the center that cause the reflection of light.

It shows that the experiment is designed properly.