How to check if game was started once

In my game I have main menu with the “New game” button.
I need to check, if I press this button first time in this game session like:

I start new game with this button, then from the game I come back to the main menu, then I press this button again.

public void ButtonNewGame()
    if() // if the game was started once
        GameObject gameMAnagerObj = GameObject.FindGameObjectWithTag("GameManager");
        GameManager gameManagerScript = gameMAnagerObj.GetComponent<GameManager>();
        gameManagerScript.currentActNumber = 0;
        act_0.stepNumber = 0;

The easiest way would be if you create a game folder in the user’s documents folder and write a file there. Then every time you click on the button you check for its existence.