The standard test for a slipping clutch is to place the front bumper against an immoveable object, like a tree; put the emergency brake on tight; put the transmission in its highest gear; and rev the engine while releasing the clutch as if you are starting off. If the engine continues to run after the clutch pedal is fully up, the clutch is slipping. If the engine stops with a “boomp”, the clutch is frictionally healthy.
If you can find it, look up “Clutch Burnout” on Youtube. A the owner of a ZO6 Corvette is trying to “roast off the rear tires”. The engine is reving; smoke is coming out all over; but the rear tires are not turning.