Try comparing the size of the individual threads in the twill pattern. Because 16oz. Is thicker thread, the same number of threads will make a wider stripe. I.e. A six-thread red stripe on a 13oz. Tartan will be narrower than a six- thread red stripe on a 16oz. Tartan, assuming the fabrics are from the same mill( to minimize other variables.). Also, a person experienced with tartan will probably be able to tell you which mill wove your fabric based on the particular shades of colors used. Then it's easy to see what weight is available from that mill in a particular tartan. All that being said, if you handle enough tartan, your eyes and fingers will "get smart". Try to get your hands on some swatchbooks. Sometimes just spending time with a mill's fabrics will lend insight.