Wrapping material (2) utilized by a packaging machine (1) is advanced along a set feed path (3) through a station where an accessory material (12), typically adhesive, is applied in spots (13) to selected areas of the surface (9), whereupon the material (2) is charged electrostatically at a first station, and thereafter sensed at a second station to reveal the distribution of the charges; the electrostatic charges register more strongly on and around the spots (13), so that by measuring any variation in strength relative to the remainder of the surface, identifiable with faulty application of the adhesive, substandard material (2) can be detected before it reaches the wrapping stations of the machine.