Arguably the most likely way we will first discover alien life on another planet will be using the power of atomic spectroscopy.
Aliens will most likely leave a tell tale trace of their life in the atmosphere’s of their planet. But how do we know what chemicals the atmosphere of a distant planet contains? The answer is atomic spectroscopy. If we see a planet passing in front of it’s star, some of that starlight is absorbed in a very specific pattern called an atomic absorption spectrum. Each element has a specific pattern like a barcode, so through careful analysis of the light it can tell us which gasses are in the atmosphere and their proportions. We already use this technique for other space objects like stars and nebulae, measuring properties like temperature, density, ionization and relative velocity. This is a gift that nature and quantum physics has given us and the majority of what we know about the universe is based on this technique.