[RPG] What skill would you use to determine a scent

Miscellaneous tricks that should make a Wizard (or in most cases, anyone) move more silently:

• Get a Collar of Umbral Metamorphosis (Tome of Magic). Grants a move silently bonus as the least of its abilities, the others include Hide in Plain Sight.
• Get some Silent Moves armor for up to +15 circumstance to move silently. Use ASF lowering techniques for best results. Alternatively, take Craft Wondrous Item and craft a custom item of +yes to move silently - if your DM is cool with that kind of stuff.
• Make that armor out of shadow silk (Tome of Magic) for an additional +2 untyped bonus. Add a Muffling armor enhancement (Underdark) for another +2.
• Buy a (cheap-ass) Rod of Silent Spell. Successfully sneaking only to call the horde of guards by speaking abracadabra "in a strong voice" is just embarrassing.
• Buy a Ring of Silent Spells (Magic Item Compendium). Even cheaper than the Rod of Silent Spell, and it grants you your own personal field of Silence that you can cast through, although to a limited degree. Keep the Silence cake and eat it too.
• Alter Self into something like a Whisper Gnome (Races of Stone) for a move silently bonus. For shorter excursions, Polymorph into something even better. Like a cat. That sounds stealthy.
• Don't go in yourself. Summon something stealthier than you and have them do stuff. A 10th-level Wizard can cast Summon Undead V (Spell Compendium), which allows summoning Shadows.
• Apply Softfoot powder (Complete Adventurer) to your boots. +1 alchemical bonus to move silently. Yes, it's +1, but you don't have an alchemical bonus already that overlaps.
• Heroism up your skill checks. Why not? Lasts forever anyway.
• Deafen your opponents. Bwahahaha.

Also,Teleport/Dimension Door/Benign Transposition (Spell Compendium) with your familiar whenever you can rather than sneaking like a muggle. Seriously, the good part about being a Spellcaster is that you can get stuff done without playing by rules like "I need to move through space to get places". CatLord's answer has additional suggestions of this nature.

Note: All of my terms and page references are from my English PDF of CoC 7th Edition. In addition, I have directly dealt with the player behavior described, and I have run CoC, but I have never dealt with this specific problem while running CoC. First I will address the rules as written. Then I have some more general advice based around things I've tried which worked in other games.

The Rules As Written

There is no rule in the 7th edition which expressly forbids players taking turns on certain tasks in order to maximize the chances of success. There ARE rules which can help you remain in the spirit of the setting.

The section When to Roll Dice beginning on page 82, and the section Rolling Dice beginning on page 194 both provide some guidance here. There's too much for a full quote, but the the core of it is "There's no need to roll dice for everything," and "The Keeper decides when to roll dice."

Applying this here, first, you as Keeper will decide when a roll is appropriate. The players can ask, and if allowed and failed, they can attempt to justify Pushing the Roll. But you decide if one is allowed in the first place. More importantly it means you can decide when a roll isn't needed at all.

Player: "We search the shelves for clues."
Keeper: "A quick search reveals one book hastily replaced out of order, its cloth bookmark marking a specific page."

No roll. Just a successful search.

The Rolling Dice section also suggests rolling in full view, even for the Keeper's rolls. I think this creates a different dynamic than rolling in secret.

The rules for Spot Hidden on page 76 address this directly, suggesting automatic success for players performing "a thorough search" might be appropriate.

The rules for "More than one player rolling dice for a skill roll" on page 86 should help you decide when you allow multiple skill rolls from the group. One of those examples expressly states all characters in a position to potentially spot something get a roll. But those rules also give you some idea when separate rolls should be allowed, and even required in some circumstances.

Lastly, if you or your players have access to the Investigator Handbook, the Rules Advice beginning on page 216 has some useful information on how to approach the CoC mindset, including this tidbit on Accepting Failure:

Don’t be disappointed when you don’t win every roll. Accept failure—it can take the story to unexpected places. Sometimes, in hindsight, you might be very grateful your investigator didn’t manage to open that cellar door.

General Advice

The rules for Pushing the Roll (I think this is what you referred to as "forced rerolls;") begin on page 84. These rules DON'T refer to multiple players, but I reference it here because they lay the groundwork for something you can try, which I have done in other games. Consequences.

When Pushing, the player must explain what gives him the impetus to make a new attempt, and then the Keeper must explain the consequences of failure. As Keeper, you could use this even when multiple players are trying. There are possible consequences to multiple attempts which could occur regardless of who's making the attempt.

• "A second complete search of the room will take time you can't afford. If you fail the Cultists will be one step closer to completing their ritual."
• "A second attempt to break down the door will definitely attract unwanted attention if you fail."
• "A second attempt to repair that engine might irreparably damage a vital part, making it useless until you can have it in a shop during downtime."
• "A second attempt to intimidate the guard may make him mad enough to sound the alarm and attack you."

Several of the above examples touch on the second point. Your players should always consider time. Time is a HUGE factor which will work for or against the players. Wasting time on multiple attempts at the same action is certainly one way to ensure time works against them. The enemy may be closing in... or getting away...

In play, I've simply made players aware of the time required to perform repeat attempts. Sometimes I mention a specific possible consequence, but others I just say something like, "Sure, you CAN all search in turns. Are you sure you want to take an entire hour?" Then, after warning them, let them do it if they want. And make it matter. "You find that piece of information that eluded you, but now you hear a key in the lock of the front door!"

Properly used, time can help you.

There's an old text-based computer adventure where all the events are timed. It's literally possible to "wait" through the entire game and the adventure will just pass you by. I'm not suggesting anything this extreme, but if you demonstrate the world isn't static while they're busy, it will encourage players to move along.

Finally, consider the necessity of the roll. The absolute best way to discourage re-rolls is to not require rolls in the first place, except when needed. We touched on this above as it related specifically to the rules, but it's something to consider all the time. Is a roll really necessary?

In play, I generally call for a roll when the result will be interesting no matter how it comes out. Failure can move a story along just as well as success. And failure can be just as boring as success in some cases.

As a corollary to this, I use a rule I borrowed from another game called "Say Yes or Roll Dice." As GM in most games, it's my job to let players have their way, OR to make a conflict out of it. I love to RP and interact, but at some point the players will ask, "look, is this guard going to get out of our way or not?" And then I will say "yes," or the scene will become a conflict and the dice will decide.

As a final note which isn't really GMing advice, just make sure your players are up for the game you're running. Your descriptions make it sound like you have a conflict of tone vs expectations. There is a... pressing darkness... an urgency implied in many situations of CoC. The looming horror isn't going to wait around for your team to make 6 separate attempts to do ANYTHING. So, what are your players expecting out of this experience?