Image downsampling

I'm sort of curious -- there are a lot of filters and masks out there, but I have heard very little about automated or algorithmic image detection, insofar as being able to, for example, determine that a portion of an image has been downsampled -- or to detect if an image has been obviously corrupted, without needing to visually preview the image. This sort of capacity would be quite useful, but I can't seem to find any references to that or any closely related topics when I research on Google. Anyone with some image analysis experience have a word or two on this subject?

If the problem is just to identify highly quantized JPEGs, read http://en.wikipedia.org/wiki/Jpeg paying special attention to the section on quantization.  You should be able to tell the degree of quantization just by reading the stupid file.