In addition to the cryptography, digital trace, and secure storage/chain of custody answers already provided, also consider the possibility of statiscial analysis that can identify manipulation in subtle ways.

Here is a link to an article that describes some of these analyses:

https://articles.forensicfocus.com/2013/08/22/detecting-forged-altered-images/

This article discusses how simply opening and saving a file can change the structure of a picture without producing visual differences; how inserting a new image into another picture will leave mathematical artifacts that can be discovered with error analysis; and how subtle differences in quality in different portions of the same photo can be proof that the image was manipulated.

Now, there *may* be techniques that can be used that *might* mitigate some of this:  one idea that comes to mind is to perfectly model a scene, and then create a perfect CGI with no artifacts.  Even in a case like this, however, it may be possible to show statistically that the scene is fake:  digital algorithms themselves often leave certain statistical "markings" that might be identifiable -- and, for that matter, going back to the quality issue, such a generated scene might have a quality that doesn't match the available video equipment that allegedly filmed the incident (ie, grainy when the camera would have had a high amount of detail, or highly-detailed when the camera would have been grainy).

Finally, it would be important to corroborate video evidence with details from the real world.  A forger, for example, may have known that the individual in question wore a popular designer jacket, but would not have been aware of the tear in the jacket that happened at a party the night before, in a spectacularly embarrassing incident that everyone saw -- or, alternatively, that photo that the prosecution is claiming to be fake also happens to show a name-tag sewn on the jacket, that the person alleged to have faked the photo would have no reason to know actually existed...