It's actually not even that challenging for them. Every photo taken by every physical camera (mirrorless, dslr, smartphone, digital point & shoot, etc) generates a file type called an EXIF. It includes metadata from the phote including colour & light information needed to display the photo, the camera information and settings (shutter, aperture, ISO, lens data) date/time & location data (if the camera has GPS capabilities).
Camera manufacturers are already working on and in some cases (Nikon) close to releasing firmware that utilizes encryption keys to make EXIF more secure and reliable. Currently, it's quite easy to alter or fake exif data. When that's the standard and it's used widely, social media platforms just need to read the metadata (which they do already, but for their own purposes like knowing what's in the photos you post so they can sell you better ads) and use the appropriate verification key for the right camera manufacturer.