This should probably be moved to the Hobie 18 forum.
To answer your question, I don't know about there being "little if any quality control" in the early years. I think some aspects of the boat design were probably controlled better than others. The hull layup itself has always been scrutinized and that is apparent in the various weights of the hulls throughout the years. But the hull alignment is controlled by molded parts (the hull deck), cast parts (the corner castings), and extruded/machined parts (the crossbars), so all of this is likely to be well controlled.
Anyway, you can certainly check the alignment of your hulls by using a tape measure and measuring the distance from one bow to the opposite stern and then taking the equivalent measurement on the other side and seeing if the measurements are the same.
The problem is that if the measurements are within about 1/4" or so, it probably doesn't make much difference and if the measurements are out by more than 1/4", there isn't a whole lot you can do about it anyway. If the hulls are out of square, you can try bedding the crossbar saddles in epoxy and pulling the boat square with your mainsheet blocks, but it is a lot of work for probably not much gain.
On a 1981 H18 that is going to be raced, I'd be more concerned with the hulls being soft and/or heavy. I would check the entire boat for soft spots and cracks under the hull flange. And I'd keep the hull covers off as much as possible (maybe even put a small fan inside each hull) to get the hulls as dry as they can be.
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