Though the theoretical uniqueness of string theory would in principle allow the determination of all parameters of low energy phenomenology, today the string theorist’s ability to predict experimental results is non-existent.– Richard Dawid
This sentence might need clarification. A direct experimental test of string theory, i. e. a test of the extendedness of elementary particles, is far beyond the grasp of current experiments. Due to the theoretical uniqueness of string theory however, the mere fact that the theory is correct in principle would determine all low energy phenomenology of the world modulo a possible degeneracy of the string potential’s energetic minima. Therefore it would be possible in principle to check whether all parameters of the microscopic world we know, from the mass of the electron to the number of particle generations, correspond to an energetic minimum of the string theory potential. If that was actually the case it clearly would constitute an overwhelming proof of the significance of string theory for nature, even if no one had ever seen a string directly in an experiment.