Its not so difficult, just take 10 mtr. Strap and check its weight on digital weighing scale and compare the result with standard specification.
Example: Standard weight of 12mm strap (0.4mm thickness) is 30 grams per 10 mtr. Now, if the weight of same size & thickness comes to 36 grams, it means 20% length is less and actual cost is 20% higher.