It's amazing how much you can infer from the shape and size of the various features of a bone.
For eyesight, simple physics: bigger is better. That's why we build huge telescopes, they collect more light and have better angular resolution than small ones, and the same goes for eyes. In addition, birds in general have very good eyesight and dinos are very closely related to birds. For T. rex, they also have narrow snouts allowing for excellent binocular vision.
Smell is similar—big nasal cavities allow for big olfactory organs, meaning a lot of receptors that can bind with airborne molecules.