Take the "slam dunk" theory of evolution. If "Some people and animals are homosexual" was in there, I'd pick that as the lie without even looking at the other two (well, if I didn't already know).

A rationalist ends up being wrong sometimes, and can only hope for well-calibrated probabilities. I think that, in the absence of observation, this is the sort of prediction that most human-level intelligences would end up getting wrong, and I wouldn't necessarily assume they were making any errors of rationality in doing so, but rather hitting the 1 out of 20 occasions when a 5% probability occurs.