Suprisingly, I cannot find the hex value that identified a .json as a .json in any online list. What is the file signature for a .json file?
JSON (nominally) doesn't have a file signature.
File signatures (usually "magic" byte sequences with given offsets) exist in binary file formats; this includes executables, images, video, music, other media files, as well as numerous proprietary file formats.
The purpose of file signatures is to provide a would-be API for identifying the type of hard-to-parse or unparseable files, which is normally the case with binary files.
JSON is data structured in a specific format, which is almost always stored in a plain text file, which can be parsed with relative ease, and so doesn't need a file signature. The closest thing would be a MIME type, which usually only applies to JSON files being sent over a network.