Measuring assembly with a measuring apparatus and a recognition system, and recognition method to detect the correct positioning of a removable device such as a comparator to check the shape and/or dimensions of a mechanical part with respect to a stationary part that includes at least one mechanical reference for the removable device. The system comprises a field source disposed in the stationary part that generates a field and a matching element disposed on the removable device that is able to alter the propagation of this field if correctly positioned relative to the field source. A sensor fixed to the stationary part in an appropriate position detects or not the field and generates a resulting detection signal detection or non-detection signal. A processing unit receives the signal generated by the sensor and on the basis of this provides information about the correct or incorrect positioning of the removable device with respect to the stationary part. The recognition system can be of the magnetic or optical type.