If Lanefan was your GM, they probably would ask your character to roll for a Perception check and then follow it up with an Investigation check to help you if possible to fill in the gap. So you roll a d20 twice, once for your Perception check and then again for your Investigation check. There would be several outcomes from these two skill checks.
1. Both your Perception check and your Investigation check succeed.
2. You succeed with your Perception check, but fail your Investigation check.
3. You fail your Perception check, but succeed at your Investigation check.
4. You fail at both skill checks.
Lanefan at this point would then describe to you what the resulting skill checks look like to your character, and then take the narrative down a particular direction within the adventure. And you would base your next action on whatever your character learned during this particular encounter.
Truth be told, I wouldn't do this.
I'd ask "What do you do?" and narrate according to their stated actions. Even though there's an NPC present, it's still a passive (or passive-looking) scene as the NPC isn't (yet) doing anything; so the PCs' actions are the only determinant of what happens next.
If their answer is that they will take a longer look without entering the room, then they'd get the bits about the flies, that the man is breathing but may or may not be awake, a fuller description of the parts of the man they can see (e.g. hair colour, general build, rough guess as to age, etc.) and of his clothing, and so forth.
If their answer is to enter the room and look around they'd get even more. And if they actually lift the man's head off the table and take a good look at him they'll then be able to determine if he's who they're looking for (let's for these purposes say he is), and go from there. Or if he's faking being drunk, their touching him is his cue to act, and out comes the hidden gun....
But if their answer is "Eeew, what a stench! Close the door quick!" they'll get none of that description.
Or worse: if the GM just assumed our actions would be to examine the man and room more closely and just proactively gave us all the info as if we had done so, without first giving us the choice as to what/how to examine (or to not examine at all and instead go check another room).
How is any of this "realistic" or "immersive"? Why is it realistic that I notice the hole in the wall, or the sweat, but not the flies? Why is it realistic that I notice the man's sweat, but not his general build, or his likely age?
Likewise, how does any of this pertain to the GM "just assuming" certain actions?
@Lanefan's description assumes that the PCs look at and take note of features of the wall, but that they don't look at and take notice of flies in their immediate visual field. I mean, how is it supposed to be the case that
I've identified the ceiling is 8' high and yet I haven't noticed the flies my eyes must have passed over in order to take in the ceiling?
I can see bottles on the floor under the window. The window is opposite the door (on the "far wall"). Presumably the man slumped forward in the simple wooden chair (but with no table to lean on? Why isn't he sliding off/down? Has he been nailed to the chair by the people who killed him?) is between me and the window, so how am I even seeing those bottles?
I don't see any realism here. It's all just
gameplay: the GM makes a decision to dispense some information and to withhold some other information; the GM takes for granted that the players will infer some things from what is not said (eg will infer that there are no aliens with rayguns in the room because the GM didn't mention any); and the GM likewise takes for granted that the players will know - in general terms - what information might have been withheld, and hence needs to be asked about. (Or, perhaps, gated behind dice rolls.)
Now if Lanefan told you that the man before you was truly drunk and not faking it all, that he was a town guard, and that he was armed with a gun, you wouldn't need to make any skill checks. Your character would just know. What then? What would you do with the knowledge that was just handed to you? I don't think I would be happy or satisfied if the info was just handed to me without my character making any effort to get it on their own.
The sort of gameplay implicit in
@Lanefan's set up - which I've described in some earlier posts, and also just above - is not very interesting to me. If the highlight of a "gumshoe" game is
asking twenty questions of the GM to get a description of a sweaty man in a room, so that the play experience is not dissimilar to poking a Gygaxian dungeon room with a 10' pole, then count me out.
What's interesting about the room is the man in it; and what is interesting about the man is whether he's dead or alive, and whether or not he's the person I'm looking for. The GM is able to dispense that information. So why not do so? Either "You open the door, into a poorly furnished office. There's a man sitting, slumped, in the simple wooden chair. Flies are hovering about and above him. You can't see his face. He looks like his dead." Or "You open the door, into a poorly furnished office. There's a man sitting, slumped, in the simple wooden chair. Flies are hovering about and above him. He barely stirs in response to you, but from his breathing and his sweat you can see he's alive."
Now the players can engage with the man, or check out the room, or whatever they want to do, without having that interesting stuff gated behind a bizarre dance of the seven veils as to the basic set-up of the scene.