OK, I get some of those symptoms simply from my throttle lagging because my car is (somehow) running rich (and lean at the same time, go figure) because of my poorly set-up de-cat. How do I tell if it's the clutch or the throttle lagging? Would I just KNOW? Would I definitely feel the difference and know something's wrong?
There is a huge difference between a slipping clutch and throttle lag.
With "throttle lag", you press the throttle and nothing much happens and the car starts to accelerate (it may take a while).
With a clutch slipping, you press the throttle, and the engine immediately speeds up (increase in RPMs) but the car doesn't immediately start to accelerate. If you pack off on the throttle, the RPMs drop back down. We're not talking small changes here - for instance, you are at 3,000 RPM, and give it gas, and the tach goes to 5,000 RPM
right now but the speedometer doesn't quickly change. Then if you back off, the RPMs drop back down to 3,000 RPM.
You can simulate a slipping clutch at any time - next time you start off from a light, only let the clutch out half way - the engine will scream, and the car won't do much of anything.