it is usually this :

1, attach line to the trap wire so it can hang down enough to reach the point where the bridal attaches to the hull (remove the shock cord first); swing the line forward
2, Hold you finger at the point the line hits the hull (or some other "class agreed point")
3, swing your hand back untill the "mark / finger" bisects the hull aft; measure this relative to some fixed oiunt (rear beam; transoms etc). This gives you a "rake measurement" which can then be compared.