Usually, the way to build a fiberglass boat is from the outside in. Basically, spray gelcoat onto the waxed mold. Then lay down chopped strand fiberglass mat. Over that, you begin your layup schedule of woven fabric (all with resin, of course.)
The purpose of that first layer of chopped strand mat is to prevent print-through in the exposed gelcoat, where you can see the pattern of the woven roving in the gelcoat surface.
In older boats, it is common for gelcoat to wear on flat surfaces, or perhaps the gelcoat was sprayed a bit thin in that area. In these situations, you will see a pattern of short, fine "cracks" oriented in many random directions. It sort of looks like a random diamond plate pattern.
On the other hand, you can get longer stress cracks, particularly along radiused edges, where the gelcoat may have pooled in the mold and went on thicker than necessary, and thus more prone to cracking. Further, corners in fiberglass must be radiused, because fiberglass doesn't like to make sharp bends. And sharp bends lead to stress risers which lead to more cracks.
Andrew's got some excellent points about trying to determine where the cracks came from...
