Error level analysis (ELA) re-saves a JPEG image at a given quality and shows, pixel by pixel, what changed. Areas compressed as many times as the rest stay dark; a pasted or edited area reacts differently and shows up bright. Paste the address of a Lab image or choose a file.
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In real life: FotoForensics, Forensically, Ghiro. ELA is not proof on its own (sharp edges and text always react a little): look for an area that stands out from similar areas in the rest of the image.