When sourcing stretch denim, the composition of elastane (Spandex/Lycra) is the most critical factor; typically, 2% to 5% elastane provides the ideal balance of comfort and shape retention. You must evaluate the 'Growth' or 'Permanent Set', which measures how much the fabric fails to return to its original length after stretching—high-quality denim should have a growth rate of less than 5%. Additionally, check the weight (measured in Ounces/oz): 8-10 oz is suitable for summer/lightweight jeans, while 12-14 oz is the standard for premium, durable denim.