Collection of works - roof trusses

Before the opacity of the roof to check that its structure is made according to the design (dimensions, sections, parts, and combinations thereof, the angle of inclination of the rafters, etc.). It's good to see the wood, which is to be performed rafter.
Assessment of the technical design of the roof truss is a layman's a very difficult task. Lack of technical expertise and knowledge of regulatory and technical recommendations in this area means that if there is a conflict with the contractor, the investor usually stand to lose. How to guard against such a situation?