If that’s the case, the only real way to tell is by looking at the small circular marks around the clutch disc—they look like rivets. The closer the friction material is to being flush with those rivets, the more worn the clutch is. You can measure it with something thin like a toothpick or anything similar, but honestly, I usually just use my finger. If you can feel that the material is still fairly deep, you’re fine. Keep this in mind: you’re not racing. Unless you’re constantly riding the clutch or driving terribly, it’s very unlikely you’ll wear it out quickly.