It looks like the heavy white foam. If you don't have a scale, then you can't check the weight of one sheet of foam and you cant check the all up weight (315 g | 11.11 oz) of the plane. The only thing you can do is to check the center of gravity with two fingers again. It is 50 mm behind the leading edge of the wing. It is also marked in the plan. I hope that your Kwak will not be too heavy for the first flight. Have fun!