Most of the time when a picture or video has been faked or photoshopped, you can probably tell if you look at it carefully. Their usually isn't something quite right, about it, that most people will miss.
For example odd lighting. If you superimpose an image chances are you do not have the lighting just right.
Picture Fragments. Sometimes if you look at photoshopped pics (Even professional ones) you might find extra or removed limbs or fingers. Or some impossible feat of a part of the body that somehow is in front of something that couldn't possible be.
Extra Sharp or Blurry: Sometimes thing of interest that is added in later is taken with better skills than the background so you will see a blurry picture with a sharp object. Or they will cover up the whole picture by making everything blurry. If the image seems like it was taken from an iPhone but it was super blurry more than what the device does you can probably expect it has been altered somehow.
Dithering/Anti-Alias methods: Most digital cameras on full resolution tend to have some dithering to the colors (Those sparkly bits that don't seem to exist in real life) Then some equipment scales it down a bit and adds some Anti-Aliasing to make the colors more smooth and natural looking. If you add a fake element chances are those methods will be different. Say a smooth well anti-aliased pipe, with a dithered person.