A probe head (1) of an NMR-MAS assembly has a stator (2) with an opening (4) receiving a rotor (3) which, in a measuring position, rotates at the magic angle to the B0 field. The stator is pivotable between the measuring position and a loading position of the rotor. A detection device (5) permits external, contactless identification of whether the opening of the stator is fitted with a rotor. The detection device has a light source (5a), from which light is introduced into a lower end (6″) of a light guide (6). The stator has a first bore (2a), in which a first light guide stump (7a) is positioned such that, in the loading position of the stator, it produces an optical connection between a rotor inserted in the stator and an upper end (6′) of the light guide opposite the lower end.