Soul is the living entity that resides inside the body and works through it. It’s a spiritual spark which is part and parcel of the Supreme Lord and is eternal, and full of knowledge and bliss, just like the Supreme Lord Himself. Lord Sri Krishna describes the nature of the soul in detail in Chapter 2 of Bhagavad-gita. Many times we say that there is a soul within us or within our bodies but in fact we ourselves are the souls and we are residing in the body. Soul is very minute to be visible to our naked eyes or through any scientific device. Its size is mentioned in the scriptures to be one ten-thousandth of the tip of hair.
As the soul is infinitesimal in size, one cannot perceive soul in the conditioned state of life. The only way to understand about is by hearing from Guru, Sadhu or Shastra. The symptom of presence of soul is consciousness. When we see consciousness in a living being then we can understand that the soul is present in the body. This consciousness or the presence of soul is the difference between the living and the dead.
Lord Krishna says in Bhagavad-gita 2.17,
avinasi tu tad viddhi
yena sarvam idam tatam
na kascit kartum arhati
That which pervades the entire body you should know to be indestructible. No one is able to destroy that imperishable soul.