When video or audio evidence is submitted, it will be questioned as to its authenticity. Who recorded it? On what device? Then we'll look for other corroborating evidence. Are there other videos that captured the events in the background of the evidence video? Are there witnesses? Is there contradictory evidence?

Say there's a video depicting a person committing murder in an alley. The defense will look for video from the adjoining streets that show the presence or absence of the murderer before or after. If those videos show cars driving by with headlights on, they will look for corresponding changes in the luminosity of the crime video. If the crime happened in the daytime, they will check that the shadows correspond to Sun's position at that moment. They'll see if the reflections of objects match the scene. They'll look for evidence that the murderer was not at the scene. Perhaps a neighbor's surveillance camera shows they were at home or their cell phone indicated they were someplace else.

But if all these things indicate the suspect was in the alley and the video is legitimate, that's powerful evidence toward a conviction.