All they really need to do is have lights on the plane roughly the same shade as the color of the sky, which is pretty easy to do. They even did this in World War 2.

The way to check it is pretty interesting. If there are any sightings people need to get hold of some 3D glasses, or any colored filters like red, green, and blue. If they are projecting the color of the background its likely they are doing this by using a combination of red, green, and blue lights to get the right shade.

So if for example they are using green in the mix and you look for them with a green filter they should stand out as green lights in the sky. If you use a red filter they should stand out as red lights and so on.