Patterns often specify that you use a fabric with a certain percentage of stretch. Easiest method of measuring % stretch? Use a metric ruler!
Place 2 pins 100mm apart. See how far your fabric will comfortably stretch. To 120mm? That's an extra 20mm or 20% stretch. Stretches to 150mm? The extra 50mm means 50% stretch. Get the idea? If it stretches to 200mm that's an extra 100mm or 100% stretch.
This is also a good time to check recovery as well. If your fabric returns to it's original size, it has great recovery. If it stays a bit stretched, it has poor recovery and you could have problems with your garment stretching out of shape over time.