When sourcing linen, the primary quality indicator is the GSM (Grams per Square Meter), which determines the fabric's weight and transparency. For apparel, a range of 150-200 GSM is standard, while upholstery requires 250+ GSM. You must also verify the yarn count (e.g., 14s, 21s, or 40s); higher numbers indicate finer, softer threads. Ensure the supplier provides OEKO-TEX Standard 100 certification to guarantee the absence of harmful chemicals, and check for colorfastness to washing (Grade 4 or higher) to prevent fading in retail products.