brahma jagad-gurur devaih
vainyasya daksine haste
drstva cihnam gadabhrtah
padayor aravindam ca
tam vai mene hareh kalam
amsah sa paramesthinah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Lord Brahma, the master of the entire universe, arrived there accompanied by all the demigods and their chiefs. Seeing the lines of Lord Visnu’s palm on King Prthu’s right hand and impressions of lotus flowers on the soles of his feet, Lord Brahma could understand that King Prthu was a partial representation of the Supreme Personality of Godhead. One whose palm bears the sign of a disc, as well as other such lines, should be considered a partial representation or incarnation of the Supreme Lord.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
There is a system by which one can detect an incarnation of the Supreme Personality of Godhead. Nowadays it has become a cheap fashion to accept any rascal as an incarnation of God, but from this incident we can see that Lord Brahma personally examined the hands and feet of King Prthu for specific signs. In their prophecies the learned sages and brahmanas accepted Prthu Maharaja as a plenary partial expansion of the Lord. During the presence of Lord Krsna, however, a king declared himself Vasudeva, and Lord Krsna killed him. Before accepting someone as an incarnation of God, one should verify his identity according to the symptoms mentioned in the sastras. Without these symptoms the pretender is subject to be killed by the authorities for pretending to be an incarnation of God.