We don't and can't. Because we can't even come up with a universally accepted definition, there can be no bright-line test.

Coupled with our innate arrogance, where we allow ourselves to "just know that we are", just like we are pretty sure that we get to exert "free will", you end up with a lot of sloppy thinking. I'm not claiming to have any answers (I'm more of an intentionally extreme skeptic of the answers I come across), but I don't think you can deny that there is a lot of sloppy thinking (esp. on a layman's board like this) around "consciousness", "intelligence" and "free will".