One believes that one will recognize that person when they meet. The meeting should have such resonance that no doubt will remain.
In current usage, “soul mate” usually refers to a romantic partner, with the implication of an exclusive lifelong bond. It commonly holds the connotation of being the strongest bond with another person, romantically, that one can achieve. It is commonly accepted that one will feel ‘complete’ once they have found their soul mate, as it is partially in the perceived definition that two souls are meant to unite.