Yes, it is technically visible in the sense that the user can show it by
clicking on its title in the task bar of by moving the other windows
around. A hidden window is a window that is really invisible to the user
and that the user can not see whatever they do.

No easy way to do that i'm afraid. As a starting point, i would suggest you
to look at the APIs in the user32 lib (you'll need to do some interop to
access those APIs). You should be able to retrieve the list of all current
windows, which should allow to retrieve their current visible state as well
as their size and location. What i don't know though is how you could get
the Z-order of the currently visible windows in order to determine whether
your window is visible to the user or not. But there might be an API for
that as well.

There is a saimple way if you don't mind trapping your form's Paint
event by looking at the ClipRectangle:

private void Form1_Paint(object sender,
System.Windows.Forms.PaintEventArgs e)
{
    string now = DateTime.Now.ToString("hh:mm:ss");

    if (e.ClipRectangle.Width > 0 || e.ClipRectangle.Height > 0)
    {

        this.Text = now;
        Debug.WriteLine("Partially visible" + now);
    }

}

This code updates the form's caption if any part needs redrawing and
hence is visible