It's all about presentation.
I'm a substance over style sort of guy. For one thing, all I hear is you saying fundamentally the same thing. The nuance in your presentation, I don't get.
In any event, if this becomes a question of what you should actually say, it's neither "Yes." nor "No." A good DM should recognize that the real problem here is that the player requesting to know exactly what is over his head through a solid object has become disengaged from the fiction. So the real thing that you should say should be something in fiction to try to redirect the player back into the fiction. Meanwhile, you should be trying to figure out why the character is disengaged from the fiction, which I would guess has something to do with player confusion about why they are in the sewers in the first place.
I'd probably say something like:
"Staring up at the roof of the sewer, you see only an unbroken arch of bricks held in place by what looks like some sort of concrete. Cockroaches cling to the bricks by the dozens. Attempting to bore through the bricks with your eyes reveals nothing. The roof could be 5 feet thick or 5000 for all you can tell by looking at it. Around you there is nothing but a slow moving stream of grey water about 12" deep, with fecal matter leisurely drifting back down the way you came. To the sides there are 12" clay pipes at intervals of about every 30 yards and intersecting the main passage near the top. A thin stream of water or urine drizzles down from the nearest one. Everything reeks of ammonia and waste leaving you short of breath."
I'd cut that shorter depending on how much I'd already told them, or elaborate in a different way about the details of what they see. In this case, there is no real need to say, "Yes or no." You just narrate the consequences of their action.
There are all sorts of ways to interact with this scene:
a) You could attempt to listen for sounds coming from the ceiling or the side pipes.
b) You could analyze the feces.
c) You could probe around in the water for trash.
d) You could ask how long ago the bricks might have been laid.
e) You could cast an information gathering spell.
f) If the player has 'scent', 'deep sense' or other unique sensing abilities, you could prompt your DM for further information about the odors to see if you can discriminate anything other than the smells of usage, or remind your DM that you can 'feel' how far below ground you are (if he's forgotten).
g) You could beg for divine intervention.
The burden is on the players to tell you how they might learn something. Once they have a plan, then you can ascertain whether they might learn something, which may or may not involve a skill check.
If the players are inept or inexperienced, you may need to prompt them with suggestions. If the players are bored, you may need to prompt them to remember the clues they have that they aren't putting together, and you might need to scan your sewer encounter table for ideas that might advance the story out of its current rut. That could be almost anything. If your party has a druid, a wandering rat becomes a possible information channel. Or, if the PCs have a retainer/henchmen, it might be time to open up a dialogue with the party through the NPC, "Remind me. Why did we come down into this retched sewer again?"
I like that my players get to contribute to the story of the game, otherwise, I feel like I'm reading them my novel and I get bored quickly.
Players contribute by proposing courses of action and asking pertinent questions about what they see. Inept players have to be trained out of their passivity, incuriosity, and reluctance to engage what surrounds them. Ironically, novice players - and particularly young players - rarely have these faults. 'Grown-ups' are more likely to be the ones that have forgotten how to play.
Sure, you need to reign in outrageous ideas, but even outrageous ideas have a kernel of plausibility somewhere in them.
Plausibility might not even be the most important point. What interests me most is why a player is offering outrageous ideas, and what sort of precedent I might be setting by going along with it. Does he want more spot light? Does he want his cunning or creativity validated? Is he just not thinking things through, or forgotten a key point? Is he bored and trying to make his own entertainment? Have I failed to set the scene appropriately? Is there something important in my mental picture I failed to mention. Is there a key point I've forgotten, like say an iron grate in the ceiling through which you might catch glimpses of the town. Is the current brainstorming likely to go anywhere? Is he upset and deliberately trying to wreck play? Am I putting improper focus on something that doesn't deserve it? Is the player acting irrationally because for some reason he thinks this is what I 'want' him to do? The real answer isn't "Yes" or "No", it's the answer to questions like, "What do I do to get my players to understand and properly interact with the scene that they are in? How can I recapture the players attention? Can I get the game moving again in a way that doesn't break suspension of disbelief?" The real problem with an outrageous suggestion is that the player has clearly become disconnected from the character and the scene, and probably has gone mentally entirely verbal and doesn't 'see' what you've described or imagine themselves being in it.
Usually my response is going to be trying to force the players imagination back in game. Show don't tell. I'm only going to go with an out of fiction answer like, "Yes" or "No", and an explanation if things are going terribly wrong.
