An AI doesn't need to be sentient to exhibit behavior that looks like genuine motivation or is equivalent to striving for survival.
At this point we are pretty sure LLMs have no "motivation". Motivation requires self. And while we don't know what self is[1], we do know that LLMs don't have it.
Also it's not even necessary, it's sufficient for it to be aligned with human values of survival (likely in its training) and act accordingly.
Having said that, your concern is technically correct: we don't. Because we don't know what conscience is, we cannot provide a definition against which to test whether it conforms or not.
Sort of like when Terminator robots will be exterminating the human race they probably won't be technically killing us. Just, like, reprioritizing world's water from humans to datacenters, all of it. But not killing people, no.
I mean. Don't need rogue AI for this. These are already the policies of those in charge of it: human needs are secondary to power and profit.
So you know what. Maybe a rogue AI will be an improvement.
If such AI decides that energy spent toward human agriculture (biological food that the AI does not need) is less important than work spent toward storage and production of energy (electricity that the AI needs), then why wouldn't it just try to re-direct resources from the former to the latter. And the AI is some sentient superintelligence, I think it is safe to assume that it will be able to outmaneuver human safeguards.
Obviously that is still just a very hypothetical sci-fi scenario, but the consequences could be very dramatic.