The first time we see adult Elijah, he's talking about a comic book artist drawing villains with disproportionately-large heads. The way the scene is shot, we only see his reflection in the picture he's describing. Elijah's hair makes his head look oversized.
In the same scene, he also says superheroes often have a square jaw. In this film, the hero is played by Bruce Willis, and near the the end of the film, the artist's rendition in the newspaper (based, presumably, on descriptions given by the girls he rescued) gives him a very impressive Lantern Jaw of Justice.
During the conversation where Elijah asks David about his ability to detect evil-doers by sheer intuition, only in retrospect do we realize David has seated Elijah in the yellow section, which is one of the colours that shows up around evil-doers.
David's Kryptonite Factor is drowning because the increased density of his bones makes him less buoyant than other people, and thus he tends to sink and drown easier.
The fact that "Mr. Glass" is exactly the sort of name a comic book supervillain would have may not be apparent to some viewers, even after the Twist Ending.
The first time I saw the movie, I, like David, assumed that his powers didn't always work after M. Night's cameo as a drug dealer who manages to give off a false positive for David's "Crime Vision." However, after a recent second watch, I noticed the bright colors of people's clothes for the criminals. The drug dealer was wearing a red and blue jacket when he grabbed the package from the trash can. When David confronts him, he's wearing a solid blue jacket. He switched clothes.
Despite being unbreakable, David apparently did get knocked unconscious by the train crash. This seems like it might contradict his being invulnerable to injury ... except that direct impact (e.g. a blow to the head) isn't necessary for a high-speed collision to render a person unconscious, as merely having one's brain shaken hard enough to temporarily scramble the signals between its neurons can have that effect. Unbreakable, yes; unshakable, no.
There have been numerous accidents in history with only one survivor, this doesn't prove the survivor is a superhero, as the villain in the story would like to believe.
Mr. Glass knew this. Being completely uninjured by a train wreck, displaying mild psychic aptitude towards human iniquity and the ability to perform feats of superhuman physicality are probably what set him apart from the rest.
Paper cuttings in his office during the reveal show that he is aware of this. Such as the one referencing a lone survivor of a mud slide.
Elijah gave David a fairly thorough grilling about his Nigh-Invulnerability in their first meeting along with a healthy dose of skepticism at the start, which suggests that David may not be the first person approached and there is a long list of qualifications besides simply surviving a disaster. Elijah had the idea but it took the entire length of the movie for both to become convinced.
David has the power to sense people's wrongdoings just by touching them. Perhaps this explains why he's drawn to his wife? He is shown being very intimate with her on several occasions (lifting her from the car wreck, carrying her to bed, nuzzling her after a "nightmare"). He never sees any misdeeds, so she must be ethically pure and she complements his own sense of justice.
Or that he loves her despite any misdeeds, as a deleted scene strongly hints that she either was having or was considering having an affair.