We can't detect immortal souls. We can't detect any sign of souls at all. As has been said, there are living human beings who show less evidence of sentience and less personality than a pet cat, rat or dog.
Some religious people are afraid that, if humans don't have souls, then people are not worth any more than animals, or insects, or dirt. There is an assumption that non-soul humans will be mistreated. As if the only reason to value a person's life is a supernatural one...Strange, how the belief that people had immortal souls did not prevent slavery, genocide or gender oppression.
I don't know of any atheist who thinks that people should be treated badly, because they don't have immortal souls. It means that we are all part of the natural world, and our existence is limited. It means that the earth is not just here for us. And when an organism dies, it is gone forever. If anything, it means that we should be treating the animals and plants of the world better, not that we should be treating people worse.