Soul is the essence of being. You are a being-- the evidence is overwhelming.
As the soul by its very definition cannot be defined within the material world, it is up to our conscience, our metaphysical selves, to define it.
In the material world, we CAN look for its "footprints": imagination, love, our desire to give, unexplainable perceptions, and the way we relate to certain people and events.
The word "soul" is vocabulary, a symbol. And it is limited as such. I prefer to define it as something that feels like it can go far beyond self. It's a notion that seems to have no beginning and no end; and is not tied exclusively to any individual. If you have ever experienced that sensation, there is some of YOUR evidence.
Best wishes in your soul-searching!