The thing about a belief is, is that it can't be proven. If one strongly believes in God, would you ask him if he has researched it, and see it proven? No.
All i can say is that i watched my guineas being outdoors for more than a decade and tried my best to interpret their behaviour and body language. That's all anyone can do in the matter of determining whether an animal is happy or not. You simply never know for sure, that's why i specifically used the word 'believe', instead of 'know'. Had i said I 'knew'
cavies are happier outdoors, you would have had every right to tell me i would need some proof for it.
You might not believe a cavie is happier outdoors, and I do. No one will ever know for sure.