dnd 5e – When a player character with Lucky is attacked, what information is given to the player before deciding to use the dowry?

A player of mine has just taken the feat Lucky (PHB, page 167), which is a very nice feat, but some questions have arisen.

Lucky's dowry allows a player to use a lucky point in the attack rolls that happen against them. The second bullet says:

You can also spend a lucky point when an attack roll is made against you. Roll a d20 and then choose if the attack uses the attacker's roll or yours. If more than one creature spends a lucky point to influence the outcome of a roll, the points cancel each other out; no additional dice are thrown.

The bullet point above indicates that when the player is attacking, he may reroll after his roll, but before the results are determined, the second block does not contain that information.

Here is the first bullet:

Each time you make an attack roll, a skill test or a save roll, you can spend a lucky point to roll an additional d20. You can choose to spend one of your lucky points after rolling the die, but before the result is determined. You choose which of the d20s is used for the attack roll, the skill test or the save roll.

So, the player must decide before the DM launches the attacks against the player with the Lucky feat? Or should the DM roll but do it where the player can see? The assumption is that he must decide before I tell him he's hit, but if I say I just got a "XX", the player knows his Armor class but may or may not know the Monster's "hit" bonus. for what may or may not be "determined."

Here is the crux of the confusion, the player has to decide to use his luck roll or the attackers, so (again another assumption) the player has some knowledge of the roll … then when that knowledge is given to the player? Or it's just a guess and they do not know the roll.