You can try to do json.loads(), which will throw a ValueError if the string you pass can't be decoded as JSON.

In general, the "Pythonic" philosophy for this kind of situation is called EAFP, for Easier to Ask for Forgiveness than Permission.

The correct answer is: stop NOT wanting to catch the ValueError.

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

import json

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

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

You can use try except clause to check if a string is actually json:

import json
line = '<what you think is json>'
try:
    json_line = json.loads(line)
except ValueError:
    print("not a json")

In your above code the last line is not a valid JSON. You can use this tool JSONLint to verify if your JSON is a valid JSON.

More compact way should be like this. Json lib process only str, bytes or bytearray structures, so only consider them. Instead of if len(text)==0, if not text is much faster for long strings, we don't want to know length of text. Json lib may raise JsonDecoderError. First last characters of text can be checked back-reference with regex but I tried possible edge cases such as '{]' and '[}', they don't fail.

def is_json(text: str) -> bool:
    from json import loads, JSONDecodeError
 
    if not isinstance(text, (str, bytes, bytearray)):
        return False
    if not text:
        return False
    text = text.strip()
    if text[0] in {'{', '['} and text[-1] in {'}', ']'}:
        try:
            loads(text)
        except (ValueError, TypeError, JSONDecodeError):
            return False
        else:
            return True
    else:
        return False

EDIT: We should check whether text is empty or not, not to raise IndexError.

def is_json(text: str) -> bool:
    if not isinstance(text, (str, bytes, bytearray)):
        return False
    if not text:
        return False
    text = text.strip()
    if text:
        if text[0] in {'{', '['} and text[-1] in {'}', ']'}:
            try:
                loads(text)
            except (ValueError, TypeError, JSONDecodeError):
                return False
            else:
                return True
        else:
            return False
    return False

first, your input isn't json. Json uses double quotes. But suppose you successfully loaded it with json, it's now a dictionary, called d.

Then you can scan all sub-dicts of d and test serial key against your value, stopping when found using any and a generator comprehension:

print(any(sd['serial']=='00000000762c1d3c' for sd in d['device']))

returns True if serial found False otherwise.

import json
try:
  json.loads(data)
except ValueError:
  print("data was not valid JSON")

You are checking data as a dictionary object. When checking using if check in data it checks if data object have a key matching the value of the check variable (data.keys() to list all keys).

One easy way would be to use if check in data["players"].__str__() which will convert value to a string and search for the match.

If you want to make sure that check value only checks for the steam64 values, you can write a simple function that will iterate over all "players" and will check their "steam64" values. Another solution would be to make list of "steam64" values for faster and easier checking.