There is nothing standard to do that - we aren't generally interested in
how a user entered the data, but
what he ended up with.
You can do it: handle the KeyPress event for the TextBox and count the key presses:
private int pressed = 0;
private void myTextBox_KeyPress(object sender, KeyPressEventArgs e)
{
if (char.IsLetterOrDigit(e.KeyChar))
{
pressed++;
}
}