A player of mine just took the Lucky feat (PHB, p. 167), which is a very nice feat, but some questions have popped up.

The Lucky feat allows a player to use a luck point on attack rolls that happen against them. The second bullet says:

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

The earlier bullet point states when the player is attacking, they can reroll after their roll but before the results are determined, the second block does not contain that information.

Here is the first bullet:

Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20. You can choose to spend one of your luck points after you roll the die, but before the outcome is determined. You choose which of the d20s is used for the attack roll, ability check, or saving throw.

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

Here is the crux of the confusion, the player has to determine to use their luck roll or the attackers, so (again another assumption) the player has some knowledge of the roll.... so when is the player given that knowledge? Or is is just a guess and they don't know the roll.

It is up to the DM how much information to give the player

There are no rules telling the DM either to show or not to show the players dice rolls. Lucky also doesn't specifically say what information is provided to the player. So this is a case where the DM would have to choose their preference and agree how the Feat would interact with that.

Jeremy Crawford confirms that this is intended to be left up to the DM:

Q: Does the Lucky feat allows the character to see the number on the enemy's die attack roll (made against him) BEFORE spending a Luck Point?

A: The Lucky feat allows you to spend a luck point to replace an attack roll made against you. That roll must occur for you to be able to replace it. The DM decides whether you see the number on the die. Typically, a DM tells you the total of an attack roll.

He says it again here.

Obviously this will greatly affect the usefulness of the feat depending on where your DM falls along the spectrum, so the best thing to do is to talk with them before taking it and see how they will treat it. Then decide if you want to take it.