Point a camera at the sun and you'll get streaks, ghosts, and a haze that washes out contrast across the whole frame, and you'll get essentially the same pattern whether there's a roll of film behind the shutter or a digital sensor. That's the tell. Halation and bloom both depend on what's recording the image — bloom is not halation covers how those two differ from each other, one happening in the film base, the other in a digital sensor's photosites. Flare doesn't belong in that comparison at all, because it happens before the light ever reaches either one.
Where Flare Actually Comes From
A lens isn't one piece of glass — it's a stack of elements with air gaps between them, and every one of those air-to-glass surfaces reflects a small fraction of the light hitting it, even with modern coatings designed to minimize exactly that. Point a bright source into the lens and some of that light bounces between elements multiple times before it finally exits toward the sensor or film plane. Each bounce path that makes it through traces its own ghost image of the aperture, offset and scaled differently depending on which surfaces it reflected off. That's what you're seeing as the repeating polygon shapes strung across the frame in a strong flare — each one is a record of one specific reflection path through the lens's own internal geometry.
The aperture blades themselves also draw a signature into the image, separate from those internal reflections. A more distant point source diffracts at the edges of the aperture opening, and the number of blades (and whether they're straight or curved) determines how many points a resulting starburst has. None of this involves the film or sensor in any way — it's a property of how light bends and reflects on its way through a specific physical lens, finished before it ever reaches the plane that's going to record an image.
Why the Coatings Matter
Lens coatings exist specifically to suppress this. An uncoated air-glass surface can reflect several percent of the light hitting it; a well-designed multi-layer coating can push that down under half a percent per surface. With ten or more such surfaces in a modern zoom, the difference between coated and uncoated stacks compounds fast; it's why older or poorly coated lenses flare so much more aggressively than current ones aimed at the same light. The coating doesn't change where flare comes from. It changes how much of it gets through.
This is also why flare is a lens property you can look up, not a camera-body property you have to guess at. A specific lens produces a specific, repeatable flare signature — the same ghost pattern, roughly the same starburst shape — regardless of which body it's mounted on, film or digital, because none of that geometry lives downstream of the lens.
Telling the Three Apart
The fastest way to sort out which of the three you're looking at is to ask where in the optical path the stray light would have to originate. If the effect changes character between a film body and a digital body using the same lens under the same light, it's happening downstream of the lens — halation if it's warm and film-specific, bloom if it's a hard, symmetric overflow specific to a digital sensor's pixel wells. If the effect looks identical regardless of what's recording it, and traces back to shapes that match the lens's own aperture and element count, it's flare, and it was already finished doing whatever it was going to do before the light reached the recording medium at all.
That distinction matters practically. You can swap film stocks to change how halation looks, or accept that a given digital sensor's bloom behavior is fixed by its hardware. Flare only changes if you change the lens, add or remove a hood, or change the coating — nothing about the film or sensor behind it will touch it.