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Question: At what distance is one able to investigate objects and/or their surroundings with the intent of identifying/finding illusions?

Context: The party is in a cliché illusion-filled dungeon. They know there could be illusions around, but aren't particularly skilled in INT or Investigation. When it's important, and they want to check before a big move, or in a pressured situation like combat, how far away can they expect to detect an illusion?

Specifics: The illusions are all being made via Major Image by a bunch of Level 13 casters with 20 INT, so the DC is (8+5+5) = 18. They are being cast at level 6 so they are "permanent." All illusions were set up before the party arrived, and all enemy casters are elsewhere, thus they have no knowledge of where illusions may or may not be. Assume the rooms/halls are all well-lit, and the party is otherwise unhindered by curses/spells/effects. The party's best Passive Investigation is too low to spot the illusions, even with advantage, thus requiring manual checks and crossed fingers.

Two cases to consider:

  1. The party see an object/creature at XXXX feet away. They wonder if it's an illusion, so they Investigate it. At what distance is this check reasonably possible, and at what distance would the DM be fair in responding to any roll with "You don't see anything abnormal about it from here"?

  2. The party is in an endless, straight hallway. They think the floor or walls may have a passage or pit concealed by an illusion, so they investigate. There is no object they're checking in detail; just a "search" of the surrounding area's walls and floors. If it was within 5' of them, and they rolled well, it seems unarguable they'd spot it. At what distance is this not going to benefit them?

Expectation: Looking for a RAW answer, but I'm assuming there isn't one. Therefore an educated suggestion, perhaps based on existing mechanics or similar activities, will work in the absence of RAW. Also, in addition to "how far can it work," it may also be "how far does it work without disadvantage, and then how far with disadvantage?"

Considerations: In terms of non-RAW, I had considered using a 60' limit, based on the ability of those with Darkvision to see their best within 60' of themselves. Darkness is NOT a factor for this question, but it's an example of using a sight-based precedent.

Please Avoid: This is not a question of good DM etiquette. By this I mean that, if the party were thinking the hallway had secret traps/passages, and the illusion is actually 200' away, a good DM would likely allow them to travel carefully, checking as they went, and they'd potentially spot it before they plummeted into it or passed a corridor. No "gotcha" involved. They could also do cool tricks like rolling ball bearings down the endless hallway [for some distance, not important to this question] and seeing if it fell in a hole. But this question is for technicality - suppose the wizard wants to Dimension Door down this endless, uniform hallway 500', and that just so happens to be exactly where the pit trap is. Then it matters. That's the intent behind asking the question. You may assume the players are given every opportunity to be successful without nit-picking.

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  • \$\begingroup\$ None of your players has even a measly +3 in Perception? None of them? Not one divine caster or even just a dude with 12 Wisdom and proficiency at level 1? No rogues or anyone else with expertise? No one with the guidance cantrip? That seems exceedingly suicidal-- illusions are super easy to spot pretty much no matter what, if you can't even see those how do you think you'll fare against legitimately difficult perception threats like ambushes or traps or enemies hiding in combat? \$\endgroup\$ – Please stop being evil Jul 17 at 1:28
  • \$\begingroup\$ @thedarkwanderer I think that even accounting for passive investigation the distance question would remain valid, although such poor perception for an entire group is a bit uncommon. \$\endgroup\$ – SeriousBri Jul 17 at 10:46
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    \$\begingroup\$ @thedarkwanderer Those [terrible] stats were chosen to rule out the impact of alternatives so that we could focus on the core of the question. Quality of a group like that is a whole other issue. :) \$\endgroup\$ – Mister B Jul 17 at 16:32
  • \$\begingroup\$ @MisterB Okay, fair enough. \$\endgroup\$ – Please stop being evil Jul 17 at 17:03
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There are no specific rules for detecting illusions besides what is in the spell description:

A creature that uses its action to examine the image can determine that it is an Illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the Illusion for what it is, the creature can see through the image, and its other sensory qualities become faint to the creature. (emphasis mine)

To investigate a situation, I would think it has to be in "sensory range", ie. visible and/or smellable and/or hearable. The player also would need to be specific about exactly what/where he is investigating.

If they get near enough and actually touch it, it is revealed as an illusion as the spell says:

Physical interaction with the image reveals it to be an Illusion, because things can pass through it.

In your particular case those are strong illusions (DC 18), so I'm guessing it will still be a challenge even if they succeed more than half their checks, which would surprise me if they are as bad at investigation as you said.

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