A unit for preparing an opening device for gluing to a respective sealed package of a pourable food product includes a conveyor for feeding the opening device along a predetermined path, a dispenser for depositing a layer of adhesive onto a portion of the opening device to be applied to a corresponding package; and a monitor for identifying, on the opening device, the position of the layer of adhesive with respect to the adhesive-receiving portion, and which generate a signal associated with the shape of the identified layer.