The invention relates to a method and arrangement for testing an image of a display device (4) for calculating a process value, wherein a graphic card (5) is arranged upstream of said display device (4). The inventive method consists in associating an additional concrete image symbol to a measured process value received from a process valuator (1) in a visualisation unit (3) placed upstream of the graphic card (5), in detecting said image symbol by a signal analyser (6) positioned downstream of said graphic card (5), in determining an associated process value according the thus detected image symbol in the visualisation unit (3), in comparing the thus determined process value with the measured process value in a comparison unit (2) and in identifying an error in the case of a difference between said two values.