How can you tell animals are sentient? Personally I don't know enough about consciousness to make either of those calls. Are trees sentient? What about stones? What about individual atoms?

I find it fascinating that people can judge this with such authority because they equate consciousness with agency, but if we can imagine a system of agency without consciousness, can it be possible for consciousness without agency? Can you prove your personal consciousness is not just the result of a network of smaller consciousness of individual neurons, which is in turn a network of consciousness of individual atoms which is a network of consciousness of subatomic particles?

Seems silly, but it's impossible to prove one way or the other. We have no idea what consciousness is so I think it represents extreme hubris, and is potentially dangerous to think something can't be conscious, just because it doesn't seem conscious.