How would you determine if they were?
It would be nice if we could come up with some kind of Turing test that proved beyond a doubt whether something was conscious and sentient or not. But, that's not possible.
However, if you understand how something works, you can say that certain things can't be conscious because of how they work. No matter how insightful the answers seem, we know that a magic 8-ball can't be conscious because the mechanism for consciousness can't exist in something that's merely a tetrahedron floating in fluid. A LLM can't be conscious because it's merely a system for generating the next word in a probabilistic way.
With a brain, we don't understand enough about how it works to say for sure whether or not it's sentient. We can't say no, but we don't understand sentience enough to say yes for certain. But, we can operate on the assumption that sentience and consciousness exists in humans and maybe in some other animals, while also knowing for certain that the word-generating program is only mimicking consciousness.