Antimorph

You cause a creature or object you can see within range to be flooded with energy that is debilitating to Transmutation magic and similar magic.

If the target is unwilling, it must 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 other Transmutation spell of 4th level or lower, or otherwise having its form altered by a similar effect.

On a failed save, the target is forced to return to its normal form if its form is being altered. For the duration, the target's form can't 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 an additional creature or object per level of spell slot 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.