One of our magi has the Difficult Longevity Ritual Flaw. The question has come up about when this would be noticed, if at all.
Position 1: The Flaw is noticed by the magus when the Ritual is performed.
Position 2: The Flaw is noticed by the magus when it fails much earlier than expected.
Position 3: The Flaw might be noticed either time, on a Magic Theory roll.
Other positions might well be possible.
You can always argue that his master could have detected - or even caused - his Flaw. Did he tell him?
Probably in the process of explaining how longevity rituals work and are created.
I'd then add Position 4: Flaw known since apprenticeship.
Also the more information about the character the player and the character share, the easiest the play.
While looking at it from the opposite direction, the Virtue Latent Magic Ability can give a Major Virtue. This is a -2 point swing in cost for what is a story effect, similar to how Story Flaws work (where a Minor Story Flaw is effectively a Minor Virtue effect with a -2 story hook and a Major Story Flaw is effectively a Minor Flaw effect with a -2 story hook).
So having a Flaw be unknown could be perceived as attaching a -2 story hook to it. Making a Major Flaw unknown can have extremely detrimental effects on the character, since it magnifies the Flaw.
Of course if the game is a "Fast Saga" or "One Shot", Difficult Longevity Ritual is not actually a Flaw at all and should not be allowed by the SG.
It is perfectly reasonable to rule that it would be detected during apprenticeship.
Personally I think I would say it is detected when someone tries to develop a Longevity Ritual for the magus.
Since that is a quite personalized process, it would be strange if the Flaw would not be noticed during the process.
It could also be detected earlier if someone (with sufficient Magic Theory / InVi / InCo score) were to specifically check for it. (Which might very well happen during apprenticeship.)
This is a much more general question: for Virtues and Flaws that have no immediate, obvious effect, what does the character know? This includes (but is not limited to) various types of Lesser and Greater Immunity, Purifying Touch, and Malediction; some Enemies; and some Hermetic Restrictions.
Even if the effect is obvious to outsiders, the character may not realize it/believe it. Does a Humble +3 character who's always Judged Unfairly know that he has Great Characteristic x2? A character with a Delusion is by definition Deluded into thinking he is not Deluded 
In our games, the player is always allowed to come up with an explanation, however far-fetched, of how the character knows, if he wants the character to know. On the other hand, the player is also free to have the character ignorant of the V/F, if he wants. If this ignorance is going to be persistent in the face of evidence, and might cause interesting situations, then the player is allowed but not required to take an appropriate Delusion for the character.
IMHO it is very unlikely to be detected at any point in time.
Magic is not science and cannot be predicted.
Life is unpredictable to start with.
Some people start suffering from ageing earlier than others, and that is still the case with magical aid. Who knows if your bad eyesight is due to bad magic or bad luck?
I would allow diagnosis with the help of InCo(Vi) magic though. There is probably some om magophysiological trait which determines the efficacy of longevity rituals. Magnitude to be negotiated at the table.
To me it is not even clear that you notice your longevity ritual failing. How would detect it? A sudden tingling in your magic receptors? A loss of some aura of longevity? A sudden feeling of imminent doom?
Of course, you cannot hide the failure from the player, so in practice it may be better to just assume that they know and move on.
The player says the character doesn't know. I believe him.
That is the heart of the issue.
But I would say that the first time someone invents a longevity ritual for the character, that magus will discuver that he has the flaw. Inventing the ritual requires a season analyzing the target's health and physical characteristics in order to tailor the effect to them. Over the course of that study, they would discover that the ritual will be less effective than usual.
Whether they inform him of this or not is a story decision.