  Identifying the quality factor of JPEG images is very useful for applications
in digital image forensics. Though several command-line tools exist and are
used in widely used software such as \emph{GIMP} (GNU Image Manipulation
Program), the well-known image editing software, or the \emph{ImageMagick}
suite, we have found that those may provide inaccurate or even wrong results.
This paper presents a simple method for determining the exact quality factor of
a JPEG image from its quantization tables. The method is presented briefly and
a sample program, written in Unix/Linux Shell bash language is provided.
