This Python script can check if Telegram user exisrs via his username.
$ python3 script.py --username username
Open answer.json. You will see the dictionary with result key. The value under this key is an answer, if Telegram user with that username exists.
Exists - means that some Telegram user has that username.
Does not exist - means that there is no any Telegram user with that username.
Error - means that some issues with yout Internet connection or Telegram servers occured.