For the technology we have now (LLM)s there does not and will not ever exist a test that can perfectly differentiate between an LLM and any human.
We will always have LLMs that will pass the test and we will have humans that will fail. The reason for this is two fold and contradictory.
The first reason is that consciousness is just a loaded concept. It's some arbitrary category with fuzzy boundaries. One persons definition of consciousness includes language another persons definition of consciousness includes logical thought. It's a made up collection of "features" that's it, no more or no less. It's a very arbitrary set of features too... mashed together for no rhyme or reason.
The second reason is that LLMs already meet the criteria for what most people technically define as consciousness. The resistance we see nowadays is simply moving the goal posts. Five years ago people were in agreement that the turing test was a really good test. We've surpassed that and changed our definitions adding more stringent criteria for sentience. AI already meets the criteria for sentience the way we defined it from 1950-2020. Thus any test to measure sentience will always be a moving target.