When evaluating seamless fabric, prioritize material composition, typically a blend of Nylon (Polyamide) and Spandex (Elastane). For high-end applications, look for a Spandex content of 15-25% to ensure superior elasticity and shape retention. Additionally, verify the GSM (Grams per Square Meter); a range of 140-200 GSM is standard for lightweight, breathable underwear. Ensure the fabric utilizes moisture-wicking technology and has a high colorfastness rating (Grade 4 or above) to prevent bleeding during washing.