Actually, it might not be as difficult as you think. Someone with a good knowledge of music theory and some ability with perfect pitch may well be able to recognize the difference with current A440 tuning and A435. Given that ability being able to recognize that it is between either and then guessing 438hz (and being lucky) doesn't seem too hard to believe to me. That said, I'm sure I couldn't manage to even guess within 100hz.
That's about four cents, or a twenty-fifth of a semitone. I don't think I can even distinguish relative pitch at that level (although 15 cents I can do easily -- roughly the difference between a just major third and an equal-tempered one).