After thinking about uses for Mind Blank and how it interacts with a bunch of spells, I then tried to think of counters and ways someone could get past it with one problem always coming up. How do you know when someone is under its effect?

Mind Blank spell description for reference:

Until the spell ends, one willing creature you touch is immune to psychic damage, any effect that would sense its emotions or read its thoughts, divination spells, and the charmed condition. The spell even foils wish spells and spells or effects of similar power used to affect the target's mind or to gain information about the target.

It seems like since it blocks divination spells and is powerful enough to foil a wish spell, a simple Detect Magic which is a divination spell would also not work for finding it. If that is the case, then is it even possible to detect someone under the affect of Mind Blank?

After thinking about uses for Mind Blank and how it interacts with a bunch of spells, I then tried to think of counters and ways someone could get past it with one problem always coming up. How do you know when someone is under its effect?

Mind Blank spell description for reference:

Until the spell ends, one willing creature you touch is immune to psychic damage, any effect that would sense its emotions or read its thoughts, divination spells, and the charmed condition. The spell even foils wish spells and spells or effects of similar power used to affect the target's mind or to gain information about the target.

It seems like since it blocks divination spells and is powerful enough to foil a wish spell, a simple Detect Magic which is a divination spell would also not work for finding it. If that is the case, then is it even possible to detect someone under the affect of Mind Blank?