As we have no data of what can be observed if extraterrestrial life is found, we can’t train ML what to look for. We will have to imagine what would happen and directly specify that to an AI system to help looking. But it will be based on our imagination, not data.

After we have observed and ascertained extraterrestrial life enough time to collect facts, we can start using ML to tell AI what to look for. How many observations? That depends on what those observations are. All the same or all different.