It’s called jnana-chaksu - knowledge of God seen through the scriptures. You listen to the account of those sages who had face to face encounters with God.
Here are some descriptions of God compiled by the great saint, Rupa Goswami, who was a contemporary of Chaitanya Mahaprabhu, 500 years ago. He wrote down his visions in a book called Bhakti Rasamrita Sidhu - summarized in Srila Prabhupada’s English translation - Nectar of Devotion.
Included are also a descriptions by Lord Brahma and Shukadeava Goswami, recorded in Bhagavat Purana.
Srila Rupa Goswami says:
"Krishna’s transcendental form has a reddish luster in seven places—His eyes, His palms, the soles of His lotus feet, His palate, His lips, His tongue and His nails. A reddish luster in these seven places is considered to be auspicious. Three parts of His body are broad: His forehead, chest and waist. Three parts of His body are very deep: His voice, intelligence and navel. There is highness in five parts of His body: His nose, arms, ears, forehead and thighs. In five parts of His body there is fineness: His skin, the hair on His head and the other parts of His body, His teeth and His fingertips. The aggregate of all these bodily features is manifest only in the bodies of great personalities."
His complexion is dark blue/black like a monsoon cloud full of water, with a translucent quality that emits the dazzling rays of the brahmajoyti, the blazing pure white light that is the goal of impersonalists.
Krishna is beautiful at His different ages—namely His childhood (kaumara up to 5 years), His boyhood (pauganda up to 10 years) and His youth ( kaisora up to 15 years). Out of these three, His youth is the reservoir of all pleasures and is the time when the highest varieties of devotional service are acceptable. At that age, Krishna is full with all transcendental qualities and is engaged in His transcendental pastimes. Therefore, devotees have accepted the beginning of His youth as the most attractive feature in ecstatic love.
The creator of the universe Lord Brahma has described Lord Krishna’s beauty thus:
“My dear Lord, the inconceivable qualities, beauties and activities which You have revealed by Your presence on this planet cannot be calculated by any material measurement. If one even tries to imagine, 'Krishna may be like this,' that is also impossible. The day may come when the material scientist, after many, many births or after many, many years, will be able to estimate the atomic constitution of the whole world, or he may be able to count the atomic fragments that permeate the sky, or he may even give an estimate of all the atoms within the universe, but still he will never be able to count the transcendental qualities in Your reservoir of transcendental bliss, nor describe Your inconceivable beauty"
Krishna's kaisora age may be divided into three parts. In the beginning of His kaisora age—that is, at the beginning of His eleventh year—the luster of His body becomes so bright that it becomes an impetus for ecstatic love. Similarly, there are reddish borders around His eyes, and a growth of soft hairs on His body. In describing this early stage of His kaisora age, Kundalata, one of the residents of Vrindavana, said to her friend, "My dear friend, I have just seen an extraordinary beauty appearing in the person of Krishna. His blackish bodily hue appears just like the indranila jewel. There are reddish signs on His eyes, and small soft hairs are appearing on His body. The appearance of these symptoms has made Him extraordinarily beautiful."
In this connection, in the Tenth Canto, Twenty-first Chapter, verse 5, of Srimad-Bhagavatam, Sukadeva Gosvami tells King Pariksit,
"My dear King, I shall try to describe how the minds of the gopis became absorbed in thought of Krishna. The gopis would meditate on Krishna's dressing Himself just like a dancing actor and entering the forest of Vrindavana, marking the ground with His footprints. They meditated on Krishna's having a turban with a peacock feather and wearing earrings on His ears and yellow-gold colored garments covered with jewels and pearls. They also meditated on Krishna's playing His flute and on all the cowherd boys' singing of the glories of the Lord." That is the description of the meditation which the gopis used to perform.
Krishna's attractive features are also described by Vrinda, the gopi after whom Vrindavana was named. She told Krishna, "My dear Madhava, Your newly invented smile has so captivated the hearts of the gopis that they are simply unable to express themselves! As such, they have become bewildered and will not talk with others. All of these gopis have become so affected that it is as if they had offered three sprinkles of water upon their lives. In other words, they have given up all hope for their living condition." According to the Indian system, when a person is dead there is a sprinkling of water on the body. Thus, the statement of Vrinda shows that the gopis were so enchanted by the beauty of Krishna that because they could not express their minds, they had decided to commit suicide.
When Krishna arrived at the age of thirteen to fourteen years, His two arms and chest assumed an unspeakable beauty, and His whole form became simply enchanting. When Krishna attained thirteen years of age, His two thighs were challenging the trunks of elephants, His rising chest was trying to come to peace talks with doors of jewels, and His two arms were minimizing the value of the bolts found on doors. Who can describe the wonderful beauty of these features of Krishna? The special beauty of Krishna's body was His mild smiling, His restless eyes and His world enchanting songs. These are the special features of this age.