Wheel type electronic sheet material area measuring machine comprises means for emitting a test start signal, operated either manually or automatically at a particular point in the machine cycle, and test means actuated by the test start signal to cause material detecting means associated with each wheel to emit a signal normally indicating presence of sheet material for a predetermined length of travel of a conveyor surface on which the material is normally supported, the wheels running on the conveyor surface. Thus each of the signalling means associated with the wheels should, if the machine is operating correctly, emit a number of test signals. The signals emitted by the signalling means of each wheel of a group of wheels are added and compared with an acceptable range of numbers of test signals which would be emitted in respect of that group if the machine is functioning within the predetermined tolerances. A warning is given if any of the groups of wheels are not within the predetermined tolerances and the machine is prevented from measuring area until functioning correctly. Preferably similar tests are run in respect of small groups of wheels e.g. ten and in respect of a group consisting of all the wheels, e.g. overall machine accuracy.