In the near future autonomous machines will be running around with various degrees of intelligence.
Something like a current mailcart that makes rounds on clockwork is pointless to talk to. A "dumb" robot may take voice commands but using colloquial language will confuse it, and being polite is not only lost on it but reduces its efficiency.
Meanwhile, a fully sentient being will not take to being talked to and treated in such a manner.
The outward appearance of the body/vehicle is not enough to tell, since the designs will change rapidly, may be specific to the industry or location, and may be interchanged anyway: a "body" may be controlled by a dumb robot running an errend (or delivering the body to where it will be used) or driven by a telepresence human in another city, or an uploaded mind, or an AI.
The social and legal responsibilities will depend on the operator, so bystanders need to know what it is even if not in a conversation with it.
I suppose that machines will bear icons or symbols of some kind indicating that they are sentient. But I never went so far as to describe it.
We're already pretty icon-happy in the West, since computer UI's evolved since the 90's. And robots may come from the East, with development of the technology and early adoption by people who use ideographic writing systems (and brought us emoji).
So, what would an official symbol of sentients that may only be displayed by a being (not left on a dumb robot) look like? Maybe there are variations and modifiers needed too, like uniform rank insignia.
More nuanced cases might be "I have the legal and moral responsibilities of a free being and not a 3-law robot, but I don't care for (or really understand) polite language and prefer to be formal and direct". After all, intelligent and mobile doesn't mean an AI that's programmed with all the social knowledge to understand colloquial speech; it might be rather alien to us.