Antimorph

You cause a creature or object you can see within range to be flooded with energy debilitating to transmutation magic and similar effects. If the target is unwilling, it can make a Charisma saving throw. Unless the saving throw succeeds by 5 or more, you learn if the target is a shapechanger, in a beast shape, under the effects of an alter self, polymorph, or another transmutation spell of 4th level or lower, or otherwise having its form changed by a similar influence.
On a failed save, the target is forced to return to its original form if its current form is altered. For the duration, the target's form cannot be altered, willingly or unwillingly, by any spell or effect.
At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, you can target one additional creature for each slot level above 4th, and unless the target's saving throw succeeds by 5 or more, you learn if it is under the effects of a transmutation spell of a level up to the spell slot used to cast this spell.