One can often deduce the focal length, if there are somewhat knowable features in the image to compare against. Also, if there is a lot chromatic aberration an

One can often deduce the focal length, if there are somewhat knowable features in the image to compare against. Also, if there is a lot chromatic aberration and the image perspective seems somewhat corrected in one axis but not in the other, chances are it's from a disposable camera. (They often have a curved film plane to compensate for simplistic lens distortion.)