Philosophically-speaking the very concept of passing raises a fascinating host of issues, since it's such a subjective concept. Passing as what? by whose definition? to whom? in what circumstances? etc, etc, etc. It could almost be taken as a textbook illustration of the way in which 'reality' is a concept created by observers, rather than an absolute, essential, objective truth.
In this particular case, the most sensible, certainly the most practical way of analyzing Liz and Jane's situation is to look at it in purely inductive terms. In other words, they have to reason on the basis of experience, rather than pure, deductive logic (the two are often confused, incidentally: many, maybe all of Sherlock Holmes's 'deductions' actually involve him inducing from experience, rather than deducing by pure logic).
Now, there is a form of passing that the person wishing to pass can know about, which is to say their own experience of going out in the world and either being aware, or not of others perceiving them as cis- or transgender. If they can go about their daily lives without being misgendered, stared at, mocked or, God forbid attacked, then they they have 'passed' in the way that matters most to them.
If they go out every day and always pass, by this definition, then they can reasonably induce from this repeated experience that they are 100% passable ... and that percentage would be lower if it was evident that they were 'clocked', depending how often that occurred.
There is, however, another, unknowable element to passing, which is that both Liz and Jane might actually be failing to pass far more often than they suspect, it's just that people are too polite or reserved to show that they've clocked them. I frequently spot people I suspect are either TV or TS but I would never be so rude as to show that recognition in a public space, because it would be hurtful and I don't want to out them. So if I clocked either Liz or Jane, they would never, I hope, know that I'd done so.
One might then question whether this second form of passing is significant. After all, people are constantly making judgements about one another: 'he's a creep', 'her pants are two sizes too small', 'I love her shoes', 'he's gorgeous, etc, etc ... silently spotting a persons transgender comes into that category.
One might also question whether Liz and Jane need to stop being so bitchy and get a life, if that's all they've got to worry about.
And come to that, maybe I should get a life, too!!