The clearest answer we have to this in Fox's Cinematic Universe is a scene in the first X-Men film.
Mystique infiltrates Cerebro by transforming her face and eyes to match those of Charles Xavier. Specifically, she mimics Xavier's retinas. We can rule out an iris scanner because she does not mimic Xavier's irises (see photo).
Retinas are not clearly visible to the naked eye. Even if one were to view a retina through the pupil, you would only see visible surface elements. Retinal scanning is done by analyzing the unique pattern of retinal blood vessels, like a thumbprint. Unlike a thumbprint, a retinal pattern is not completely visible from outside the human body.
Since not even genetically identical individuals, such as twins, have the same thumbprints, let alone retinal patterns, we're left with one assumption:
Mystique can transform into individuals (or objects, such as a Statue of Liberty bust) on a level that doesn't require knowledge of every detail in order to replicate it. Her power must work on some other non-visual level, even if it must be triggered visually. This transformation can include include incredible details she can't see, such as retinal patterns.
However, it may be that Mystique doesn't have to see a target in order to transform into them. In X-Men: First Class, Mystique transforms into an older version of herself (as we're meant to assume, because it's played by actress Rebecca Romijn who plays older Mystique in the first three X-Men movies). It is possible that this wasn't meant to literally be herself aged in this scene, and maybe she had seen a women that looked like that. However, I think that argument makes the specific cameo of Rebecca pointless, and thus we should assume that it was meant to be her aged self.