Do you get to know if an enemy died before deciding to smite or not?

A lot of emphasis is put on Divine Smite 's trigger, but for me what is as much telling wether its a seperate instance of damage or the same one as the attack is the fact that its an extra damage from the attack.

Quote from up2ng>>

Quote from jl8e>>

Dude, that is literally sequential timing. You determine if it's a hit, then you roll damage.

Ok, we're kind of splitting hairs here and perhaps my wording for my explanation on this point could have been better.  The point that I was trying to make is that the rules establish what happens when you Make an Attack.  It's a three-step process which gets fully completed before we move on to the next aspect of gameplay unless there was some sort of event-trigger happening during this process.  Within this, one single step is listed which says that you determine if the attack hits and, on a hit, you roll the corresponding damage.  It might be useful here to fully quote the entire rule so that we can see all of the context:

Making an Attack

When you take the Attack action, you make an attack. Some other actions, Bonus Actions, and Reactions also let you make an attack. Whether you strike with a Melee weapon, fire a Ranged weapon, or make an attack roll as part of a spell, an attack has the following structure:

1. Choose a Target. Pick a target within your attack’s range: a creature, an object, or a location.
2. Determine Modifiers. The DM determines whether the target has Cover (see the next section) and whether you have Advantage or Disadvantage against the target. In addition, spells, special abilities, and other effects can apply penalties or bonuses to your attack roll.
3. Resolve the Attack.Make the attack roll, as detailed earlier in this chapter. On a hit, you roll damage unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.

Step #3 above is how an attack is resolved.  If the attack is a hit, the damage is determined and applied as part of resolving the attack.  

When a feature says that something happens "immediately after hitting a target with a Melee weapon or an Unarmed Strike", we are no longer talking about any moment that occurs within the resolution of the attack.  We are talking about after that event has occurred. 

If the trigger was meant to occur during the attack in question, then the authors would have used a phrase such as "when" or "during", not "after".

Quote from jl8e>>

The writers of D&D 5e have never set down in print anything resembling a complete timing model. 

Perhaps someday the writers might go into detail about the differences between "when" and "after" in a future Sage Advice entry. 

Quote from SunIsGettingRealLow96589 >>

Quote from jl8e >>

 Not really. It's a matter of style -- how much information one normally gives the player.

It's also a matter of how one does the mechanics. If you're expecting the player to roll all damage dice at once, it's entirely possible not to know if they're dead or not before the player commits to the smite.

Its a matter of style, all right. The kind i would advise any player to run, dont walk, away from.

It's a bit of Roleplaying versus Rollplaying. You can give indicators as to the health of NPCs without giving numbers or having everyone act at 100% then drop. Some DMs will tell players the numbers or exact/rough percentages. Some will use descriptive language to increase or maintain the immersion of the encounter. You lose a lot of tension and drama if the DM is just going to tell you there's no point. Instead of wasted PC resource, that's a wasted opportunity for the DM to highlight that final blow and describe the over-the-topness. A missed opportunity to translate that into demoralizing the remaining allies or inspiring the PC's allies. DMs with the right style will use thei imagination to play beyond the numbers and craft an interesting story.

In pickup games, you get what you get. For extended games, discuss the DM's style at session zero and plan around it. I would say that the rules have been laid out thoroughly, but it really comes down to the DM and before you count on any strategy, discuss it with the DM and make sure it works the way you expect at their table. It can be fun when a spur of the moment plan comes together and surprisingly works, but if you have a tactic in mind ahead of time, there's no merit in not letting your DM know. The only reason to do so would be to blindside the DM and pressure them into making a decision they wouldn't otherwise make. I like to think that the days of DMs versus players are generally behind us.

Quote from SmiteMakesRight_3_5 >>

[...] I like to think that the days of DMs versus players are generally behind us.

 This should be the way, IMHO.

As a DM, it's not on my mind to take advantage of the rules all the time just to prevent players from enjoying their features or having a good moment during the game.

The replies here have made me wonder the following: If Divine Smite and Shield are interpreted to both happen immediately on a hit, can the target of the attack choose to cast Shield after the attacker chooses to cast Divine Smite?

Quote from Frachese >>

The replies here have made me wonder the following: If Divine Smite and Shield are interpreted to both happen immediately on a hit, can the target of the attack choose to cast Shield after the attacker chooses to cast Divine Smite?

 I'd follow this ruling:

Quote from jl8e >>

[...] Secondly, they do have different timing. Shield interrupts hit determination. Smites are post-hit-determination. You can't wait until your opponent activates their smite to say "well, then I'll shield". (Probably. It's certainly how I'd play it, but without a deep dive I can't be bothered to do, I'm not convinced it's actually 100% clear-cut. Anyway, that way lies madness. This is not an invitation to debate that question.) [...]

Oh yeah, I missed that part among all the long posts.

Quote from Frachese >>

Oh yeah, I missed that part among all the long posts.

 No, no problem at all! It's a tricky interaction, to be honest.

Even weirder: If a creature hits, chooses to use their bonus action to Smite, and then the target castes Shield, which causes the initial attack to miss, does the attacker get their bonus action and Smite back - since the condition for using the bonus action and Smite are no longer met, and therefore, could not have been used....

As a DM, since I play D&D, not Magic: the Gathering, I would generally always err on the side of the player. If the PC were the target of the attack, I would give them the option to cast shield after the successful hit; but, if they decline, and the attacker triggers the Smite - that opportunity has passed, and the PC takes the extra damage. This is no different than if the attack also caused some other effect triggered by a successful hit (conditions such as prone or poisoned, for example) - I would not allow the PC to retroactively cast Shield once the effects of the successful attack have been announced (nor would I expect to be able to do so as a player).

If the PC were the attacker, I would interject with the target's decision to cast Shield as soon after the attack was successful, and hopefully be able to do so before the bonus action & Smite were announced. I certainly would not require the PC to expend those resources.

Quote from Thyrwyn >>

Even weirder: If a creature hits, chooses to use their bonus action to Smite, and then the target castes Shield, which causes the initial attack to miss, does the attacker get their bonus action and Smite back - since the condition for using the bonus action and Smite are no longer met, and therefore, could not have been used....

 

 If Shield cause the hit to instead miss, i would rule Divine Smite  or any other effect triggerring on a hit to be unusable as the hit never occured.

Indeed. I think the interaction outlined there is just another reason to rule that trigger for Shield is immediately before the trigger for Smite.