Till better methods are discovered, I think a Turing test analogue is the best we can do.
Do you mean something like this?
http://www.rawstory.com/2015/12/this-picture-reveals-how-dol...
The really interesting question then becomes "is it always literal, or do they have abstract imagery too?".
If it's the latter, that is, in my admittedly inexpert opinion, a slam dunk for dolphin sentience - abstract symbolism allows art.
A common example of anthropomorphism is in voles (and other rodents). [1,2] Very long ago, scientists noticed that bank vole pups frequently cry when their mother leaves the nest. The scientists assumed that the pups were afraid, and missed their parent. Eventually, it was discovered that the absence of their mother drops the temperature of the underground nest, and that the pups cries are actually a mechanism to generate heat. When scientists took their mother away, but increased the heat of their den to normal levels, the pups did not cry. The animals were not communicating or displaying anything resembling "human" emotion, but were innately responding to environmental stress.
No matter how much we want to believe that animals are sentient, have emotions, and communicate to each other via language, science indicates that's just not the case. In fact, there have been numerous studies that indicate anthropomorphism is a byproduct of our complex social structure.
Nice job bringing racism into a discussion about dolphin vocalizations, by the way.
Then again, perhaps you are just a well trained markov chain, and not actually a person.
That's an overstatement. All you can really conclude is that science cannot say whether or not any non-humans are sentient.
I agree that human characteristics are sometimes falsely assigned to animals. But that doesn't mean that human characteristics can only ever exist in humans. If a crow exhibits theory of mind, it just suggests that humans are not as unusual as previously thought. It does not mean that we are assigning the full philosophical implications of human sentience to that animal. In fact science could not possibly do that and still be science.
Infants are known to lack forward-planning, object permanence, and many many other indications of reasoning. I don't think it'd be wrong to say that they behave quite like little finite state machines.
Human crying as any strenous activity generates heat.
This is not evidence that the crying act was just to generate heat. Not that it's evidence that they missed their mom, but it's neither evidence for the heat theory.
You can comfort a crying child with a blanket; maybe all human distress is just a need for a blankie?
Or, perhaps, "the Computer does not truly think or feel, it simply follows numerical algorithms that can give the appearance of thinking and feeling"?
(Obviously I'm not attempting to compare the intellect of Africans and that of computers; just to make the point that it's really hard to describe what is going on inside someone or something else's mind, when we don't even have clear definitions for what it means to "think" or "feel".)
Ota Benga is a nice (not so nice) case study in the impact of this sort of thinking.
(Sourced from https://plus.maths.org/content/computers-ltd ; the paper of Turing's can be read at http://web.cs.ucla.edu/~rosen/161/turing_paper.html )
We are extremely aware of our computers - every pixel it emanates from the screen, every bounce on the keyboard, the temperature, it's hum. With our awareness we build a model of the computer, we can anticipate and manipulate it's future states. The computer on the other hand, senses only touches on its keyboard, the power feeding to it from the power socket, the light entering its camera, the bits coming through on it's wifi. Sure, this information are enough to build some sort of model of the world, but it does not currently do that. Our computers do not marshal all of it's sensory experience into a coherent whole, and therefore remains in unawareness, cannot anticipate and cannot manipulate the world around it informed by its own experience, it acts and thinks like the detached tail of a lizard.
Nonetheless, I do think, one day, computers will think and be aware.
Assuming that only humans are sentient is a arrogant as assuming only the white man is sentient.
On the note of how they communicate - I wonder if anyone has considered sonar holography - I.e. The clicks and whistles aren't words per se but are interpreted visually, as an actual sonar reflection off an actual object would be. I'm trying to find the paper but I seem to recall some research in which they realised a dolphin was mimicking the sonar echo of an object, with startling accuracy.