The specified weight of any fabric, is the weight of one linear yard (or meter, depending on where it was woven) as it comes off the bolt, regardless of width.

Material from a bolt of 24" goods, that is said to be 8oz. will be the same thickness as material from a 48" bolt of goods rated at 16oz.

Without the experience to be able to tell from the feel of the cloth, weighing a yard of the material is the only way I can think of.

When bying yardage in person, if the specific weight of the fabric is not on the bolt of goods, I go by the feel of the cloth. Thick/heavier for colder weather, thin/lighter for warmer weather.