In reactive near field the electric and magnetic fields are not orthogonal and any object placed in this region will distort the antenna pattern.
This is one clue that could give information where this field stops when moving far from the antenna. For doing this, have to do a far-field antenna pattern measurements, before and after placing an object.
Second option (which is relative easier to implement) is, because in reactive near field the coupling is totally inductive, means that any changes of the impedance of the coupled device would change the impedance of the antenna.