How do you know if one life form is more or less sentient than another? Is sentience not a subjective phenomenon?

I don't think there is currently any way to know this objectively. That's why I framed it as a matter of belief. Based on what we know about the similarities of animal nervous systems (including humans) and the dissimilarities of plant nervous systems (so to speak), I personally believe that animals are sentient to one extent or another but I find it hard to imagine that plants would be.