Testing their qualification by Soter's Discriminant is observationally difficult, but classifying by Margot's Pi would be fairly easy. It will depend on their mass and orbital distance.

For those unfamiliar, Soter's Discriminant is a measure of orbital clearing by the ratio of the object's mass to the total mass of all other objects sharing the orbital space. The more the object cleared the orbit the bigger that ratio will be, and there's a pretty large gap between planets and dwarf planets. Of course, using it requires some knowledge of the population of objects in that space, which right now is rather poor that far out.

Margot's Pi is a dynamical measure of the object's ability to clear its neighborhood over the lifetime of the star. The larger the mass and the smaller the orbit, the shorter the clearing time:

So for example, if Planet Nine is at 700AU, then it must be at least twice the mass of Earth to clear its orbit over the age of the solar system.