The most obvious sign of a failing clutch is a condition referred to as slipping. Cartoq.com explains that this condition is a result of the clutch disk being worn to the point that the engine flywheel is not transferring power to the transmission effectively. As well as a "slipping" feel, engine RPMs that climb higher than normal for the travel speed of the vehicle indicate slipping.
Cartoq.com also lists shuddering, chirping or grinding noises and rough shifting as warning signs of a bad clutch. Shuddering is when the driver feels excessive shaking or vibration upon releasing the clutch pedal, particularly at low speeds. Shuddering is frequently caused by contamination of the clutch disk by substances like grease, oil or even water.
Chirping or grinding noises that come and go as the driver presses in and releases the clutch pedal are indicative of a worn clutch release bearing. Cartoq.com warns that ignoring such sounds may lead to permanent damage to other clutch components.
When a driver experiences difficulty shifting gears, even when the clutch pedal is fully depressed, it can point to clutch problems. Rough shifting usually does mean the clutch disk itself is worn, but rather another clutch assembly component, such as the clutch fork or slave cylinder, is poorly adjusted or in disrepair.