My answer would be by mentally making it tangible. Associate it in the same way a blind person associates red with the color hot and blue with cold. If you believe in an afterlife, then a soul, in essence is as tangible as the body that gave it up. Speaking from my own experience of coming into contact with a dark entity, it does not feel nice.
My family and I went on holiday to Cornwall and visited a place called Lands End. Our intention was to wait in a pub, for 45 min, until they started serving the evening meal. I had this horrible foreboding feeling that something bad had happened there. At first it was like a tingling feeling crawling up my spine. Then I became extremely anxious and fatigued. I even tried to lay my head on the table but I couldn't sit still. As time passed, my chest felt as though something were pressing down on it. I couldn't get enough air. Then the nausea came. Finally, though we only had 5 min. to wait for our meal, I told my husband I had to leave. As we drove back to our B&B, where we were staying, the more distance we put between us and this place, the better I felt.
Later, I decided to do some research and discovered that a woman, presumed to be a witch had been put into a well and left to die. Guess what? The pub was built over the top of the existing well. If fact, it was only a few yards from where we were sitting.
If a soul is bad or evil it would be described as dark and cold causing a feeling of uneasiness. Whereas a good soul would be the opposite; warm, bright, peaceful.