Along with the dimensions Benson has suggested, pics of the thwart shapes, including looking for a taper on the ends where they meet the rails is useful. good pics of the seat frames, and underside of the decks are often useful. Dont fall into the trap of rigidly looking at published measurements, the boats often differ slightly but moreover look at the overall dimensions. if its a 16' it will have two thwarts, one for 15,14 on down. Rib width and and taper is useful for dating, and the screw pattern for a keel to see if its a shoe or regular. lots of pictures are a great help.