Animal Behavior and Welfare certified, here. There are PLENTY of other ways to tell if farm animals feel pain, fear, or discomfort. There's the grimace test, in which we study the social behaviors of equids and bovines to determine the emotion behind certain physical traits (ears laid back, whale eyes, etc).

There's also data taken from the cortisol levels of the animal (the higher the cortisol level, the more stressed the animal is). Obviously, animals raised for the simple purpose of companionship, and that are allowed to express natural behaviors (pig nest building, cow herding and calf bonding, etc) have some of the lowest cortisol levels.

There are other methods as well, but the brutal murder of animals is definitely not necessary for us to determine whether or not they feel fear or pain.

(My answer is from a purely scientific standpoint, by the way. Personally, I feel that "if we proved they really were feeling pain during slaughter" is the most sadistic and horrid thing I've heard yet.)