C# winform check if control is physicaly visible

A pragmatic solution is to use the form's GetChildAtPoint() method, passing the 4 corners of the control. If one of them returns true then the control is definitely visible. It is not 100% reliable, all 4 corners could be overlapped by another control but still leave part of interior visible. I would not worry about that, too bizarre.

public bool ChildReallyVisible(Control child) {
    var pos = this.PointToClient(child.PointToScreen(Point.Empty));

    //Test the top left
    if (this.GetChildAtPoint(pos) == child) return true;

    //Test the top right
    if (this.GetChildAtPoint(new Point(pos.X + child.Width - 1, pos.Y)) == child) return true;

    //Test the bottom left
    if (this.GetChildAtPoint(new Point(pos.X, pos.Y + child.Height -1)) == child) return true;

    //Test the bottom right
    if (this.GetChildAtPoint(new Point(pos.X + child.Width - 1, pos.Y + child.Height -1)) == child) return true;

    return false;
}