Hany Farid from Darmouth published a tutorial on image forensics. Some image sensor artifacts help to recognize a forgery:
"a CFA interpolated image can be detected (in the absence of noise) by noticing, for example, that every other sample in every other row or column is perfectly correlated to its neighbors. At the same time, the non-interpolated samples are less likely to be correlated in precisely the same manner. Furthermore, it is likely that tampering will destroy these correlations, or that the splicing together of two images from different cameras will create inconsistent correlations across the composite image. As such, the lack of correlations produced by CFA interpolation can be used to expose it as a forgery."
Also, PRNU can be used to determine if an image is originated from a specific camera, or if a portion of an image has been altered.