The Wild Magic sorcerer's 6th-level Bend Luck feature (PHB, p. 103) states:
When another creature you can see makes an attack roll, an ability check, or a saving throw, you can use your reaction and spend 2 sorcery points to roll 1d4 and apply the number rolled as a bonus or penalty (your choice) to the creature’s roll. You can do so after the creature rolls but before any effects of the roll occur.
This indicates that Bend Luck can be used after the die roll in question is made. However, it is unclear whether you can use it after it has been announced whether a roll passes or fails.
As a concrete and realistic example, suppose a monster makes a saving throw against the Sorcerer's fireball. The DM rolls the saving throw. The DM declares whether the save passes or fails, based on their knowledge of the monster's stats and what the player says the save DC is. The DM then applies damage and other effects to the monster if applicable then moves on to the next action.
What is the latest point in that sequence of events when the player can use Bend Luck?
Note that if the DM rolls in secret behind a screen (as I do) then the player does not know the result on the die until the DM announces it, and even then they often only announce a pass/fail result. This can make the timing tricky if we treat it like Bardic Inspiration, which must be used before the pass/fail result is announced.
If the player must decide to use Bend Luck before the result is announced, then this is essentially the same as having to use Bend Luck before the roll was made (unless the DM publicly announces all their rolls, but that would slow things down and then what's the point of the DM screen?).
If the choice can be made after the pass/fail result is announced, then this is more useful. As a corollary, though, how should the player be informed of the die result, if at all, for them to make an informed decision about whether to use Bend Luck?