And you can’t prove something is sentient. But scientists have criteria that help determine whether a species is sentient. See this review for example.
even if you could, why should sentience matter?
I already answered. If something can’t be harmed there no need to prevent harming it.
what ethical system even accounts for sentience as a factor of right behavior?
Respect for animal welfare is often based on the belief that nonhuman animals are sentient and that consideration should be given to their well-being or suffering, especially when they are under the care of humans.[4]