Srimad Bhagavatham : 5.4.2.
When the son of Maharaja Nabhi became visible, He evinced all good qualities described by the great poets—namely, a well-built body with all the symptoms of the Godhead, prowess, strength, beauty, name, fame, influence and enthusiasm. When the father, Maharaja Nabhi, saw all these qualities, he thought his son to be the best of human beings or the supreme being. Therefore he gave Him the name Rshabha.
To accept someone as God or an incarnation of God, one must observe the symptoms of God in his body. All the symptoms were found in the body of Maharaja Nabhi’s extraordinarily powerful son. His body was well structured, and He displayed all the transcendental qualities. He showed great influence, and He could control His mind and senses. Consequently He was named Rshabha, which indicates that He was the supreme living being.