[RPG] How much information should the defender have when deciding whether or not to cast Shield

No. You get to wait to see if the attack would hit you.

It's specifically a reaction to when you were hit and it allows you to rewind and replay the attack as if you'd cast shield before it (using the original attack roll). See the text:

Casting Time: 1 reaction, which you take when you are hit by an attack or targeted by the magic missile spell

...

An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from magic missile. (Basic D&D p100)

So the clear intent here is the use it after the attack roll so you know whether or not you've been hit.

This is not meta gaming any more than 4e's immediate interrupts were meta gaming (if the action negated the trigger, then the action didn't happen but was expended). This is about modeling quick reaction in a way that's fair. It might be a bit meta, but it's far better than requiring a wizard to use a valuable spell (especially at early levels) for something that wouldn't have affected them anyways.

If you're looking for a simulationist model, this is the wizard waiting to see if the attack would otherwise hit them and only casting shield then.

Shield specifically specifies that you are "hit" with an attack. Parry specifies that you are damaged.

Generally things that allow you to use your reaction will proc on one of several conditions:

• You are targeted. You take this action before you know the outcome of the roll
• You are hit. You take this action after you know the outcome of the roll
• You take damage. You take this action after you know the outcome, and after the damage has been rolled.

With each of these the circumstances of the player DM interaction is slightly different.

With the first, the DM says "The monster targets you with X" and you say "ok, wait, I'm going to use X" before he rolls the attack.

With the second, the DM says "The monster targets you with X, his roll is 20, does that hit your AC?", you say "It would, but I'm going to use Shield, so now it doesnt"

With the third, the DM says "The monster targets you with X, his roll is 20, that hits your AC, he deals 10 damage", and you say "I'm going to use Parry, and reduce the damage by 1d6".

With Shield, it is up to the DM whether or not he reveals the total of the roll or not. I generally think it is good practice to do so, but the DM is within his rights to ask for your ACs and keep track of the changes (this also creates more bookkeeping for the DM which can be a pain, just put your cards on the table).

It is likewise, up to the DM whether he tells you how much damage is done before Parry is applied. However, in general, I'd also prefer open communication here.