How to implement/validate IsPostBack on Page_Load?

Question

public partial class _Default : System.Web.UI.Page
{
    private Random random = new Random();
    private string testFilePath = @"C:\Users\Nor  Asyraf  Mohd No\Documents\TestFile.txt";
    private List<string> testFileLines;
    protected void Page_Load(object sender, EventArgs e)
    {


            if (!File.Exists(testFilePath))
            {
                Response.Write($"The file does not exist: {testFilePath}");

            }
            else
            {
                try
                {
                    testFileLines = File.ReadAllLines(testFilePath).ToList();
                }
                catch (Exception ex)
                {
                    Response.Write($"Could not read file {testFilePath}. Exception details: {ex}");
                }
            }


    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        if (testFileLines != null && testFileLines.Count > 0)
        {
            var randomLine = testFileLines[random.Next(testFileLines.Count)];
            Label1.Text = randomLine;
            testFileLines.Remove(randomLine);
        }
    }
}