In your spawning condition you can use "not inside layout" condition to allow a valid spawn. It will then keep trying to place until it was outside layout. I have used this method on several projects and even with quite a few spawning objects its pretty fast with little overhead.
Otherwise you will have to determine the x,y values for your screen size and manually spawn outside these values.
You could cheat a little as well.. you could place "spawner" sprites outside your screen area and then randomly choose one of them to spawn from. I use this method when I want the enemy to randomly come from off screen but control their entry more precisely..