This question got me thinking about a possible test that anyone driving a manual transmission could perform to diagnose severe clutch plate slippage:

• Engage handbrake, make sure there is nothing in front of the vehicle
• Try to shift to the highest possible gear without depressing the clutch pedal
• If the gearbox doesn't make grinding noises, this would indicate clutch slippage. Depending on the severity of the slippage, it could be that some gears would grind and some wouldn't.

Would this test work?