JpegQuality estimates the quality of a JPEG by analyzing the image. The algorithm it implements is described in
Zhou Wang, Hamid R. Sheikh and Alan C. Bovik, NO-REFERENCE PERCEPTUAL QUALITY ASSESSMENT OF JPEG COMPRESSED IMAGES, 2002.
See CiteSeerX and the author's website.
Run
$ java JpegQuality <filename.jpg>
in the folder where JpegQuality.class resides.
Java Runtime Environment