I am not as seasoned as Cheechako. Below is the same method I use with random pieces of fiberglass
cloth when I don't know what weight it is:
I would weigh the sail in kilograms and come up with a square area in meters (a=1/2*b*h for a basic triangle), adding in some area for tape and corner reinforcements and taking off weight for bolt ropes, hardware
, etc..Then do the math for GSM (grams per square meter) and compare it to basic GSM numbers from a sailcloth company. Your number may not be a 100% match but should be close enough to indicate one or another cloth weight.
PS on Sailselect.com it's made mentioned of 'English' and 'US' oz/sqyd...that is why I said use GSM.