Check what @ray_sullivan said, the light trick is less reliable now. Deep fake makers have gotten better at matching lighting and shadows. It's still a good first step, but try adding other checks like watching for odd eye movements.
Skyler_Lopez said to add more checks like eye movements, but honestly how often does this even come up. I mean unless your mom is getting video calls from famous people, it feels like we're solving a problem most folks just don't have. The news makes it sound like every other post is fake, but I just don't see it. Maybe it's not that big a deal yet.