I am wondering how a person knows the clutch is not holding up under higher HP/torque...does it just "feel" like a slip or can you hear it too?

I can smell it more than anything else. It is quite distintive, at least to my nose.

You will smell it, and you will feel it.
I've told this story before, but about 3 months after I installed my EVC and pushed the boost up to 11.5 psi, I found that if I was at full boost in 3rd gear, as soon as I hit 4000 rpms, my rpms would rise QUICKLY but my speed would drop.

I made an appointment, ordered a TRD clutch from the dealer, lowered the boost and tookt he car on a 3am run to the Toyota dealer parking lot. About 1/4 mile from the dealer, the clutch gave out COMPLETELY. It was a very smelly and smokey end to that trip - to the point that a local police officer stopped and asked me questions. I am glad that there were no people on the road and that my roommate was there in his car to assist me in getting my car to the dealer.
Hindsight is 20/20; I wish I had known more about my car back them, or I would have replaced the clutch myself and saved more than $500.
The smell is very.. bad. Smells like something very corrosive and bad for your lungs is burning - at least to me.