"It is a stone used to detect magic."

And Need was created with magic; even when she wasn't actually doing something, she must be "visible" to someone capable of detecting magic.

Still, he ought to be capable of detecting magic, and he looked sincere.

She loved this spell; it detected hidden magic.

It was a spell to detect magic.

It can also be used to detect magic in use anywhere in the world and provide Strange with a location and visual.

She wore the tracking ring on a chain around her neck, near the ring that detected magic.

Jhessail sighed and cast a spell to detect magic.

Phaerimm have no eyes or ears, but they can still see and hear, as well as sense and detect magic.

I assume they have some means of detecting magic, if they are policing their kingdom of it.