Many years ago, I learned this gemara in Ketubot daf 10b.
Someone came before Rabban Gamaliel the son of Rabbi [and] said to him, 'My master, I have had intercourse [with my newly-wedded wife] and I have not found any blood.' She [the wife] said to him, 'My master, I am still a virgin.' He [then] said to them: Bring me two handmaids, one [who is] a virgin and one who had intercourse with a man. They brought to him [two such handmaids], and he placed them upon a cask of wine. [In the case of] the one who was no more a virgin its smell7 went through,8 [in the case of] the virgin the smell did not go through.9 He [then] placed this one [the young wife] also [on a cask of wine]. and its smell10 did not go through. He11 [then] said to him:12 Go, be happy with thy bargain.13 — But he should have examined her from the very beginning!14 — He had heard a tradition,15 but he had not seen it done in practice.16 and he thought. The matter might not be certain17 and it would not be proper18 to deal lightly with daughters of Israel.19
And naturally, it sounded a bit "strange." Is this really the case? There is no direct pathway between the lower regions and the mouth, such that the odor of alcohol should pass through! As such, one would suspect that this is based on the faulty science of the day.
Indeed, I think the Jewish Worker had the same analysis back in 2005:
The difficulty with the story should be obvious to everyone, we know now that this kind of test proves nothing and in fact is based on a completely false physiological premise. The fact that this story is quoted l'halacha further complicates the issue as the gemara clearly needs to be taken literally.
It seems clear to me that Chazal were relying on the medical knowledge of their day and that this test is not Torah M'Sinai.
However, neither Jewish Worker nor I have performed this test myself. So it seems a bit presumptuous to dismiss it out of hand, even if it seems crazy to us, based on our own assumptions and knowledge of the science of our day.
What brings this to mind is a somewhat risque possible urban legend that Snopes covers, about a vodka tampon. They write:
The most commonly-given reason, the avoidance of bowery breath, has the least going for it. If one were to ingest vodka vaginally (or anally -- the rumor is also expressed that way), the practice wouldn't result in booze-free breath because alcohol is partially expelled from the body via the lungs. Once liquor is in the blood, at least some of it gets breathed out, which is how breathalyzers measure blood alcohol content.
Sitting on top of a cask is not the same as somehow bringing it into one's body, but if there are fumes, perhaps they can enter the body and get someone slightly intoxicated. And it is not travelling through the body directly, but entering the bloodstream and then exiting the lungs.
And perhaps if the hymen is intact, the alcohol fumes do not have as much surface area to come in contact with, and so not so much alcohol would be absorbed into the bloodstream, such that alcohol would not be smelled on the woman's breath.
This sound like something that can be subjected to an empirical test. Rabban Gamliel the son of Rabbi was able to call for two participants for his study. But it would be interesting if we could confirm or refute his findings.
Regardless, I think that the straightforward reading of this gemara is that this was not halacha leMoshe miSinai, and this would be so whether or not this scientific "fact" is confirmed or refuted. The gemara itself said that he heard it as a tradition. In most cases of tradition which is halacha leMoshe miSinai, does one take steps to confirm the veracity? Well, in most cases one cannot, but still, how would he doubt a halacha leMoshe miSinai. Rather, he had heard a report of this, either from a scientist or from a rabbi (who may have heard it from a scientist), and so he saw fit to empirically test it. He certainly did not know it was true via ruach hakodesh!