Check the position to see if they are within the winform bounds. You can determine the bounds by creating a transform between the highest level WPF control and the winform, then using coordinate 0,0. Combined with the hosted window width and height, you now have the bounds of the window. Translate your position using a tranform to the highest level WPF control, and now you have a method of checking if the position is within the winform bounds.
After you've decided if the mouse is within the winform. Refer to this question
If it's not in the winform, use VisualTreeHelper.
public Rect HostRect
{
get
{
var transform = _Host.TransformToVisual(this);
return new Rect(transform.Transform(new Point(0, 0)), new Point(_Host.ActualWidth, _Host.ActualHeight));
}
}