how to tell if the clutch needs replacing

recently my '81 720 seems like the clutch has been slipping, because when im driving and the road starts going uphill, it slows down, but the engine rpms seem to not slow as much. someone said the clutch fork isnt letting go all the way, like as if the clutch was kind of pressed in. there isnt an ajustment on it, so it it just that it needs a new clutch kit?