The golden standard for me, about anything really, is a number of published research from relevant experts that are not affiliated with the entities invested in the outcome of the study, forming some kind of scientific consensus. The question of sentience is a bit of a murky water, so I, as a random programmer, can't tell you what the exact composition of those experts and their research should be, I suspect it itself is a subject for a study or twelve.
Right now, based on my understanding of the topic, there is a binary sentience/non sentience switch, but there is a gradient after that. I'm not sure we know enough about the topic to understand the gradient before this point, I'm sure it should exist, but since we never actually made one or even confirmed that it's possible to make one, we don't know much about it.