Map of T-rex's sinuses and airways compared to a Human
Our first look at Tyrannosaurs to conclude whether or not they were active predators, bully-like scavengers, or opportunistic feeders starts by looking at their senses. Could the Tyrannosaurid's senses actually support an active hunter, or were their minds set for finding already dead meals? So, let's get started with this debate on the Tyrant Lizard's senses:
We know from cat scans of Tyrannosaur brain cases that their eyesight was well developed. It was about as advanced as a human’s and was supported on a head about 12-13 feet off the ground, so the animal had good eyes, and a good view. However, T-rex, unlike almost every other large theropod up to that point in time, had an increased field of binocular vision and depth perception.
All other more primitive Tyrannosaurids didn't even come close to the forward facing eyes of T-rex. It by far had the most well developed eyesight in the entire group. However, does this good eyesight mean T-rex was an active hunter? Well, no.
Yes binocular vision is an important evolutionary advantage to securing prey nowadays, and is seen in many active predators. But many scavengers also have it. Vultures, condors, Tasmanian devils, Johnny Rooks, hyenas, and many other scavenging animals have binocular vision. Same thing is true of predators, not all have binocular vision. Nile crocodiles don't have it, yet they remain top predators in their environment, hunting almost everything in it.
So really, vision doesn't mean much in this argument. However, the fact that T-rex was evolving better eyesight than most of its predecessors seems to suggest binocular vision was getting more important down the Tyrannosaur line.
Smell, as with eyesight, was always known to be good in Tyrannosaurids. Cat scans and casts of the braincase showed extremely large olfactory lobes, the area in the brain that processes smell in animals. Not just that, but the large nasal chamber in between the olfactory lobes and the nostrils was huge and long in many species.
On the inside of this chamber were billions of tiny sensors. The exact number of sensors is unknown, but for comparison a human has about as many sensors as would cover a postal stamp, and a bloodhound has about as many sensors as would cover a tablecloth. T-rex, having a skull as big or bigger than a bloodhound, might have had enough sensors in its nose to cover the side of a house, but it's just an educated guess.
This was one of the many reasons T-rex was thought to be a scavenger; like a turkey vulture, T-rex could sniff out dead animals from a great distance away. However, just like with eyesight, it doesn't necessarily mean anything. Many modern day predators have great smell and use it to track down their prey. The same is true for scavengers. Not all vultures have a great sense of smell. In fact, many old world vultures don't have a very well developed sense of smell at all, but instead rely mostly on eyesight to find dead animals.
Hearing ability in T-rex was unknown for a long time since the delicate structure of the inner ear wasn't typically well preserved in fossils or casts. But thanks to recent advances in technology, we now know T-rex had very good hearing, as was expected.
Modern day birds and crocodiles are well tuned to hear low frequency noises. Pigeons can hear coming storms from more than 30 miles away. Alligators in Florida have also been known to react to the low frequency rockets launched by NASA. Most dinosaurs had equally good hearing, and the same is seen in T-rex. In fact, T-rex has also been found to have a huge Tympanic Membrane (Ear drum), more evidence for great hearing. However, again, this doesn't mean anything.
Hearing doesn't really mean the animal is a good hunter, or a good scavenger. Either predator or scavenger might benefit from acute hearing, so is doesn't support either argument.
Typically, large predatory dinosaurs have the intelligence level comparable to that of a crocodile. Crocodilians are very smart animals that can think and learn from their environment very easily. Many predatory dinosaurs were the same, but T-rex and its kin were different.
Unlike almost all other large predatory dinosaurs, T-rex was descended from small predatory theropods in the Jurassic called Coelurosaurs. Coelurosaurs are known for being small, intelligent predators that hunted small game among the shadows of top predators like Allosaurus and Carcharodontosaurus. But T-rex's ancestors evolved gigantic sizes once all those large carnivores went extinct during the early stages of the late Cretaceous and the size of the Tyrannosaur’s brain remained large.
Tyrannosaurs were the smartest of all large theropods; their intelligence probably was within the very lowest levels of bird intelligence. It could probably think and make things out to a better degree than crocodiles can today. But still, as with modern crocodiles and birds within T-rex's range of intelligence, he probably still used more instinct than brainpower while hunting.
As with all the previous senses though, it doesn't mean anything. Many scavengers today are very wily and smart and a number of predators today are not particularly intelligent such as the monitor lizard. T-rex just kept the same brain size to body ratio as its ancestors. So again, intelligence doesn't really mean anything in this argument.
T-rex and its fellow Tyrannosaurs had senses that don't really support a predatory or scavenging lifestyle. They support both. A predatory animal or a scavenging animal could get along well with T-rex's very reliable senses. So its senses don’t suggest either.