I have just bought a second hand car, and I have the feeling that the clutch is worn out.
How can I do a quick clutch sanity check?
With the vehicle stopped, and in a safe place, start out in 4th gear, and slowly release the clutch while giving the engine gas.
If the clutch is in good condition, the car should stall - if there's any slipping, you'll probably feel it, and should disengage the clutch right away to prevent further damage.
If the car happens to start going without stalling, then chances are that the clutch is good as well.
Alternatively, get to highway speed in your highest gear and floor it. If the RPMs shoot up, you've got slippage.