Matt,
Normally the plans will give you a reference point from which to measure the suggested CG location. It can be from the Leading edge of the wing, or from a bulkhead, In any case, I usually try to locate that point on the top of the fuselage. I normally put a small piece of masking tape on the fuselage and put the mark on the tape. Actually, all you need is just a dot.
As for the previous technique mentioned, using a wire through the fuse at the desired location, you give up one advantage of the sling method, which is the lateral balance. A heavy wing will show up and you can also add some weight to a tip if it appears necessary.,
Dash