Does dispel magic tell you if you fail dispelling?

[Falcon X]

If a creature has a high level, non-mind effecting illusion over themselves to pretend to be something else, and a wizard fails on their dispel check, does the wizard know they failed?

Specifically, this is a dispel magic being cast on a mind blanked creature, and the wizard thinks a dispel magic might be able to discern if the creature is who they say they are, or just using an illusion.

[DarkKnightJin]

It might depend on the DM, but at my table you'd be informed that while you attempted to end the magic affecting the target of your Dispel Magic, you feel that the magic was too powerful, and remains on the target.

[Falcon X]

What if you don’t even know if they have a spell on them at all? The wizard I’d just flinging a dispel magic to see if anything is there.

[Demonslayer666]

No, not unless you had a way to know the illusion was still there. I would have a failed dispel magic act the same as if there was no illusion to dispel. There would be no visible effect.

If you can detect the illusion somehow, then yes, you would know dispel magic failed.

[DMThac0]

There is nothing in the spell that specifically states you are aware of the spell being dispelled.

The narrative result of casting Dispel Magic, the way the DM lets players know success or failure, is dependent on how the DM feels the effect might look.

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If there is a visible effect, I narrate that there is a visible result from Dispel Magic. The subtle glow from Enchant Weapon fades away.

If there is a non-visible effect, but the target is altered in some way, the alteration is seen as being changed. A person under Dominate would show confusion and their behavior may change.

If there is a non-visible effect, and there is nothing altered, there is a feeling of the Weave releasing from the target. Mind Blank has no real effect that could be perceived except under specific circumstances, so instead there's a subtle burst of energy from the magic being dispelled.

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For other DMs there may be no visible/tangible effect in some of those instances and you'd be left to assume they were either not under a spell, or the spell was successfully dispelled.

[Keravath]

The text of dispel magic says the following:

"Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends. For each spell of 4th level or higher on the target, make an ability check using your spelkasting ability. The DC equals 10 the spell's level. On a successful check, the spell ends."

You roll for each spell effect on the target. If successful it ends. If not it doesn't. The only feedback the caster of the dispel magic would receive would be whether there is any visible change in the target. The caster could subsequently use detect magic to see if there is some magical effect still in place but mind blank makes the target immune to divination spells including detect magic so the caster wouldn't know whether the target was under the effect of a mind blank other than the fact that detect thoughts does not work.

[Maelynn]

The wizard I’d just flinging a dispel magic to see if anything is there.

If they don't know it's there, then they also won't know if it's not there.

Like stated, it depends on whether the magic altered something perceptible. If it blocked something, that becomes visible. If it created something, that disappears. That isn't something the Wizard can infer from the spell itself, but only from any perceptible changes that are a result of the absence of the magic in question.

[darknite]

I wouldn't think so. You would need to empirically determine the effect. However in play the DM asking you to roll a check and getting a 19 or better will typically indicate you have dispelled a 9th level or less spell.