It's really, really hard -- I would say impossible -- to prove that variations or changes have not been introduced since the time of a hypothetical original text, copied from handwriting scribe to handwriting scribe.

It might be hard or even borderline impossible, but I do respect people who honestly try. I know for instance, that Jehovah's Witnesses did a lot of work in cross-corelating as many different copies of the scriptures as they could get their hands on to weed out mistranslations, copy errors, etc. when developing their own translation. So for whatever it's worth, it's nice that some people at least try.