There’s a knack to spotting if an image has been edited or not. Shadows are usually the first give-away, as is inconsistant lighting. If you suspect a person’s head may have been added to another body, look extremely closely at the skin tones, and gradations. Look at the fringe, and see if the background looks consistent, colour-wise, with the original. Finally, use common sense and don’t believe your own eyes - if something looks unnatural, take a closer look, and see if you can find out why it doesn’t look kosher.