Example Python script returns a boolean if a string is valid json:

import json

def is_json(myjson):
  try:
    json.loads(myjson)
  except ValueError as e:
    return False
  return True

Which prints:

print is_json("{}")                          #prints True
print is_json("{asdf}")                      #prints False
print is_json('{ "age":100}')                #prints True
print is_json("{'age':100 }")                #prints False
print is_json("{\"age\":100 }")              #prints True
print is_json('{"age":100 }')                #prints True
print is_json('{"foo":[5,6.8],"foo":"bar"}') #prints True

Convert a JSON string to a Python dictionary:

import json
mydict = json.loads('{"foo":"bar"}')
print(mydict['foo'])    #prints bar

mylist = json.loads("[5,6,7]")
print(mylist)
[5, 6, 7]

Convert a python object to JSON string:

foo = {}
foo['gummy'] = 'bear'
print(json.dumps(foo))           #prints {"gummy": "bear"}

If you want access to low-level parsing, don't roll your own, use an existing library: http://www.json.org/

Great tutorial on python JSON module: https://pymotw.com/2/json/

Is String JSON and show syntax errors and error messages:

sudo cpan JSON::XS
echo '{"foo":[5,6.8],"foo":"bar" bar}' > myjson.json
json_xs -t none < myjson.json

Prints:

, or } expected while parsing object/hash, at character offset 28 (before "bar}
at /usr/local/bin/json_xs line 183, <STDIN> line 1.

json_xs is capable of syntax checking, parsing, prittifying, encoding, decoding and more:

https://metacpan.org/pod/json_xs

As you're using a JSON file, you can use this example:

import json
def validate(filename):
    with open(filename) as file:
        try:
            data = json.load(file) # put JSON-data to a variable
            print("Valid JSON")    # in case json is valid
            return data
        except json.decoder.JSONDecodeError:
            print("Invalid JSON")  # in case json is invalid

You can use parse_constant, documentation says:

if specified, will be called with one of the following strings: -Infinity, Infinity, NaN. This can be used to raise an exception if invalid JSON numbers are encountered.

For example:

import json

s = '{"position": NaN, "score": 0.3}'


def validate(val):
    if val == 'NaN':
        raise ValueError('Nan is not allowed')
    return val


print(json.loads(s, parse_constant=validate))