it'll start to slip and you won't get the "punch" off the line that you are used to. Pull off the clutchbell and give it a "look see." Scuff the inside of the clutch bell with 220 grit sand paper and inspect the shoes. If they seem ok and there's no significant flat spots, scuff those up lightly, relube the bearings and replace springs if needed. I would check the clutch and its components at least once a month or every 1/2 gallon of fuel.