How do scientsts determine is a newly found thing is dead or alive.

The substance is dead or alive is determined by response or consciousness of the substance to external stimulus.

The living organisms do have several characteristics. They are growth, reproduction, consciousness etc. But among them, consciousness is the only feature that's confined within only the living world. No non living things have this feature.

So if the scientist can find that the substance found have some response to external stimulus, then it does have life. And if it doesn't response to any sort of external stimulus, then it doesn't have life.