Do we know how animals refer to us, humans?
The question seem to assume that animals (as a weirdly inclusive group) have a "word" to refer to humans. The vast majority of animals species do not use any specific signal to refer to humans. Very few species has the social intelligence needed to communicate about the the presence of individuals of a difference species and among them few have important enough direct relationship with humans to have do so.

Consider vervet monkeys as an example. Vervet monkeys make sounds to indicate the presence of a flying vs terrestrial predators. If a human shows up, then the sound should be the one associated to terrestrial predator but that would not qualify as a sound to refer to humans. See Seyfarth, R.M.; D.L Cheney; Peter Marler (1980). "Vervet Monkey Alarm Calls: Semantic communication in a Free-Ranging Primate". Animal Behaviour. 28 (4): 1070–1094. doi:10.1016/S0003-3472(80)80097-2

Outside of a very small zoo population of some vertebrate species, I doubt (but might be proven wrong, who knows?!) there is any species that would have a term for "human". If you have any reason to think the opposite, can you please try to focus on this specific species for which you think individuals might vocalize (or signal in any way) to convey the concept of "human" to its congeners. Likely the only species that has a "word" for "humans" are humans!


  Do scientists know of a particular sound (or maybe a sign, a movement, or related form of communication) used by any animal which signifies "human"?


It is not a matter of knowing what sound they use. Excluding humans, animals likely don't use any sound to refer to the concept of "human".


  In case the current answer is no, is this ever possible to know?


The answer is not "no, we don't know" but rather "there is nothing to be known here as animals just don't do that."