I have yet to find a system where python -mjson.tool doesn’t work. So you can do:
python -mjson.tool "$somefile" > /dev/null
The exit code will be nonzero and you get the parse error on stderr if the file is not valid JSON.
Note: The Python libraries don’t follow the JSON spec and allow NaN and Infinity as values. So using json.tool will let some errors slip through. Still it’s good enough for my use case of catching errors in human-written documents early.