Most illusions will say what the requirements are to realise they are fake. Some are found out the moment you physically interact with them - others you can choose to use your action to make a check/save (and it's almost always intelligence).
As Emmber says, the rules vary depending on what spell is being used, and the spell description will tell you what you need to do. For example, Programmed Illusion says "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." Most spells that require effort to disbelieve have a line very similar to that one. It required an action (not a whole turn), and it's Intelligence (Investigation).
Great notes and examples provided, all spot on. Since your questions is specifically what rules interact with Illusions, I'll add that creatures with different senses like Truesight are special cases to consider as well. All useful notes to reference when considering how illusions are adjudicated in game.
All of that is true but. What would determine why they would investigate or not.
Examlple.
I create a wall to block an ally and a guard chasing me runs by. Why would the guard investigate the wall? What if he is not that familiar with the area. He runs past the wall why would he stop to take a closer look at the wall? on the other hand, the guard happend to grow up in the neighborhood and there has never been a wall there before. Now there is a reason to investigate and roll for DC.
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What are the rules for disbelieving illusions?
My DM is old school and thinks we need: 1) a reason to disbelieve, 2) to spend a full round to do a Wisdom Save to disbelieve an illusion.
Most illusions will say what the requirements are to realise they are fake. Some are found out the moment you physically interact with them - others you can choose to use your action to make a check/save (and it's almost always intelligence).
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As Emmber says, the rules vary depending on what spell is being used, and the spell description will tell you what you need to do. For example, Programmed Illusion says "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." Most spells that require effort to disbelieve have a line very similar to that one. It required an action (not a whole turn), and it's Intelligence (Investigation).
Great notes and examples provided, all spot on. Since your questions is specifically what rules interact with Illusions, I'll add that creatures with different senses like Truesight are special cases to consider as well. All useful notes to reference when considering how illusions are adjudicated in game.
All of that is true but. What would determine why they would investigate or not.
Examlple.
I create a wall to block an ally and a guard chasing me runs by. Why would the guard investigate the wall? What if he is not that familiar with the area. He runs past the wall why would he stop to take a closer look at the wall? on the other hand, the guard happend to grow up in the neighborhood and there has never been a wall there before. Now there is a reason to investigate and roll for DC.