We will never, in the lifetime of anybody who guesses we ever existed, be able to build an accelerator powerful enough to check whether it is right about gravity. So, they potter and try to show this or that family of variations (among 10^500 imagined) does or doesn't contradict details of the Standard Model we have most confidence in.
it would require being able to generate a high enough energy beam
But using current accelerator technology it would require an accelerator many times the size of the earth, _many_.
I use to work at a particle accelerator, part time, when i was in college. Fun fact i once confirmed Einsteins photoelectric effect using a high energy x-ray beam, a copper target, and high voltage.
