Here is a dressed out 20sf 1969 model. While the top of the cap is cut off you can see the gap between the outer hull and inner liner. It should sound kinda hollow when you pound on the hull sides. As you near the bow there is slightly more glass built in during construction as this is the business end of the boat. Plus the two sides are starting to join which makes it stiffer as well. As Sonny was saying it is important to make sure that the area were the liner and hull join (under the rub rail) is solid. There was very little foam in my 20sf on the sides just what you see in the picture.
strick