You can't know. You can know what acute suffering looks like and try to avoid it.
You can observe animal behavior and say, "They appear not to be suffering from existential dread beyond what they would in the wild", you may be able to measure cortisol levels in the meat. But you'll never know for sure.
You can observe animal behavior and say, "They appear not to be suffering from existential dread beyond what they would in the wild", you may be able to measure cortisol levels in the meat. But you'll never know for sure.