"The answer is that we will probably never be certain, but as long as we keep asking the questions we are serving science," OK, so TREX had an excellent sense of smell. So do most modern apex predators, like bears, wolves and big cats.
TREX also had vision oriented forward - like a predator.
Enormous legs and highly developed clawed feet. Not to mention powerful jaws and huge, CYLINDRICAL teeth. Teeth like modern big cats.
The idea that an animal that big, and that powerful, stood around WAITING for stuff to die seems ENORMOUSLY intractable.
Horner's a smart guy and all, but sustaining that much bulk on carrion alone is unlikely. TREX may not have jousted with Triceratops, but he could easily have run down smaller prey and simply devoured them like nacho chips.
Considering current estimates put T. Rex in the 5,000 to 8,000 kg range, waiting for something to die to feed it -- even if it had a lower metabolism (while it may not have been warm blooded, it may have been semi-warm blooded and thus a tad more active than your average reptile) -- seems like a bit of a crapshoot when it comes to securing the next meal for such a really large animal. While I'm sure the average T. Rex wouldn't have turned down a carcass, or snagged another critter's kill, odds are he did a bit of hunting too.