I am specifically asking about the many spells that contain the following text or similar:
If a creature uses its action to examine the [illusion], the creature can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC.
The RAW answer seems to be no, even when considering outlandish illusory effects.
A creature can be certain of himself that what he is perceiving is an illusion. However, the illusion would not appear faint until interacted with or investigated. It does not seem to me that simply by observing an illusion and deducing it for what it is, no matter how sure of himself a creature may be, would constitute interacting with it, or (especially important when in combat) does it constitute investigating it with an action. That creature would still, on some level, need to check his certainty by doing one of the previous things.
There lies an inherent uncertainty factor in illusions, no matter how positive a creature is about a particular illusions’ true nature. Being “sure” of an illusion and suspicious of one, while differing levels of certainty, seem to both not be sufficient enough to determine that illusion is fake and see through it.
Am I correct? (For those in the know, should I post this as answer to my own question or is this appropriately asked?)
Possibly related. This answerer suggests that a caster need not interact with or examine his own illusion in order to see through it. There are some ridiculous conclusions to draw here about an illusionist being susceptible in some fashion to his own spells. Perhaps casting an illusion is enough of an interaction so as to reveal it to the caster as fake.
Note that the accepted answer implies a caster may actually be susceptible to his own illusions. I digress.
Does Minor Illusion break only when someone spends an action to investigate it?
Related, though this question and its accepted answer do not fully address what I’m asking, only how much intent is necessary between an interaction and an illusion.