Our Gemara on amud beis discusses two kinds of litmus tests used to determine status and purity of young maidens.  There were two tests, each one given at different times in history:

In regard to the Moabite captives, whom were to be spared only if they were young enough to not have any sense of sexuality (which was age 3). The maidens were presented before the Golden Headplate that the Cohen Gadol wore (Tzitz Hazahav).  If the face of the girl did not turn sallow, then it indicated that she was still innocent and below age 3.

In regard to Yavesh Gilad (as described in Judges 21) who were treasonous and did not participate in the civil war against the Tribe of Binyamin, the maidens who were still virgins were also spared.  In this case, a different test was administered in order to determine their virginity.  They would be seated on a barrel of wine, and if a wine odor emanated from their mouth, they were determined not to be virgins.

Rashi (Kesuvos 10b) explains the second test as natural and not miraculous.  If the maiden’s private area was more closed, the vapors of the wine would not enter their body.  However, if they weren’t virgins, a subtle amount of alcohol could be sensed.  If you will, an ancient breathalyzer.  Maharsha explains the test as operating supernaturally.  Wine represents lust and hedonism.  The maidens who acted sinfully and lustfully, emanated this spiritual odor.  Much as the Golden Headplate activated a shameful pallor on the maidens who were less innocent.  

In reality, Rashi was not commenting on the miracle in the time of Yavesh Gilad. Rashi’s comment was on Kesuvos regarding a maiden who came before Rabban Gamliel whose husband claimed she was not a virgin while she maintained she was. In order to determine the veracity of her claim, Rabban Gamliel used this test. It is also notable that Rabban Gamliel did not just do the test on her, but he first calibrated his senses by comparison with two Servant girls who sat on the barrels, one known to be a virgin and the other known to have already had intercourse. The point being, the Gemara in Kesuvos is clear that this was a non-miraculous test.

It would seem that Rabban Gamliel knew how to use this test, or thought he could try to learn how to use this test from the incident of Yavesh Gilad. 

At first glance, it seems that the Maharsha on our Gemara disagrees with Rashi and offers a different peshat. Notably, the Maharsha does not quote Rashi in Kesuvos as if to argue with him. I believe this is because he does not think he is arguing at all, as Rashi was describing a natural procedure conducted by Rabban Gamliel, and not a miraculous procedure utilized by the Judges in the incident of Yavesh Gilad.

(However, there is a textual indication that points to the fact that Rashi believed the barrel test to be natural even by Yavesh Gilad. Rashi over here in our Gemara adds a two line comment regarding the test used for the Moabites with the Golden Headplate: “נס היה - it was miraculous”. This implies that only the process with the Headplate was miraculous.)

What compelled the Maharsha to diverge from assuming that Rabban Gamilel’s test was the same as Yavesh Gilad? I believe the Maharsha was perplexed as to why our Gemara even wonders why the judges did not choose to use the Headplate test. There is a famous principle that ordinarily we do not rely on miracles (Kiddushin 39b). Therefore, why would the Gemara question the efforts of the judges in the Yavesh Gilad incident to use the barrel test, it would have been preferable to using the Golden Headplate and expect a miracle! Therefore, the Maharsha determined that both tests in scriptural times represented a miracle. Rabban Gamliel modeled his test after the Biblical one, but he did expect miracles. That is why he had to calibrate his olfactory sense with a trial run.

If my interpretation of Maharsha is correct, we can learn another lesson from Rabban Gamliel’s behavior. Sometimes, with something that has a naturalistic basis, you can mix in segulah energies to assist. Even perhaps, we can consider the miracle itself amplifies a natural tendency, so if you are unsure of meriting the miracle, you might still derive from the miracle the presence of a natural tendency. Notice how many of the plagues in Egypt were jump-started by a naturalistic action such as throwing ash in the air, hitting the ground or the Nile, or the Midrash that Nachshon Ben Aminadav had to jump into the sea before it split (sotah 37a).

So Rabban Gamliel was unsure if he would merit the miracle but considered the barrel test as a good starting point, because perhaps even without the miraculous elements, the natural forces which acted like a catalyst for the miracle, might still have some stand alone powers. A modern day example of this might be a person with a skin condition choosing to bathe in the Jordan River, because once upon a time, it miraculously cured Na’aman of his leprosy (II Kings 5). 

I will add a personal note, that may seem strange, but you cannot argue with anecdotal personal remarks.  It is a segulah for good eyesight if you look at your tzitzis during the last paragraph of Shema (Mishna Berurah 24:7).  Also, for those who are interested, there is a school of thought that though generally one requires glasses as one ages, because the cornea gets harder and less elastic (like all other organs when you age), there are vision therapy exercises that help.  The eye has about a dozen muscles in charge of focusing, so if you strengthen those muscles, you can compensate for the rigidity of the cornea.  When I say Shema and look at my tzitizisl, I also do the vision therapy exercises, amplifying the theurgic power of the segulah.  I am 55 and still have 20/20 vision.  Just saying…