The primary quality marker is the Patchoulol content, which should ideally be above 30% for high-quality therapeutic or perfumery grades. Additionally, check the Iron content; 'Iron-free' oil, distilled in stainless steel, is preferred for perfumery as it prevents discoloration. Ensure the oil has a deep, earthy, and woody aroma without any burnt or chemical undertones, and verify the Refractive Index (1.505 to 1.515) and Specific Gravity (0.950 to 0.975) against international standards like ISO 3757.