In Hindu texts (such as Mahabharata) god is depicted with features more or less similar to Homo sapiens. So how did the rishis/munis who wrote Vedas/Purans after listening to Ākāśavāni predicted how a particular god would look?
“Think of the universe as an ocean of ether, consisting of layer after layer of varying degrees of vibration under the action of Prana; away from the centre the vibrations are less, nearer to it they become quicker and quicker; one order of vibration makes one plane. Then suppose these ranges of vibrations are cut into planes, so many millions of miles one set of vibration, and then so many millions of miles another still higher set of vibration, and so on. It is, therefore, probable, that those who live on the plane of a certain state of vibration will have the power of recognising one another, but will not recognise those above them. Yet, just as by the telescope and the microscope we can increase the scope of our vision, similarly we can by Yoga bring ourselves to the state of vibration of another plane, and thus enable ourselves to see what is going on there. Suppose this room is full of beings whom we do not see. They represent Prana in a certain state of vibration while we represent another. Suppose they represent a quick one, and we the opposite. Prana is the material of which they are composed, as well as we. All are parts of the same ocean of Prana, they differ only in their rate of vibration. If I can bring myself to the quick vibration, this plane will immediately change for me: I shall not see you any more; you vanish and they appear. Some of you, perhaps, know this to be true.”
This quoted from Raja Yoga book of Swami Vivekananda. This may be a way of seeing how the devas look.
Another thing that may happen is divya dristi. I have read about drivya drishti in Bhagavad Gita. In BG, Sri Krishna gives drivya drishti to Arjuna and then Arjuna is able to see God in his Vishwaroopa.
Another theory is that Gods themselves revealed themselves to Certain sages and rishis. This also happened in Mahabharata.
Kesava, that slayer of hostile heroes burst out into a loud laughter. And as the high-souled Sauri laughed, from his body, that resembled a blazing fire, issued myriads of gods, each of lightning effulgence, and not bigger than the thumb. And on his forehead appeared Brahman, and on his breast Rudra. And on his arms appeared the regents of the world, and from his mouth issued Agni, the Adityas, theSadhyas, the Vasus, the Aswins, the Marutas, with Indra, and the Viswedevas. And myriads of Yakshas, and the Gandharvas, and Rakshasas also, of the same measure and form, issued thence. And from his two arms issued Sankarshana and Dhananjaya. And Arjuna stood on his right, bow in hand, and Rama stood on his left, armed with the plough. And behind him stood Bhima, and Yudhishthira, and the two sons of Madri, and before him were all the Andhakas and the Vrishnis with Pradyumna and other chiefs bearing mighty weapons upraised. And on his diverse arms were seen the conch, the discus, the mace, the bow called Saranga, the plough, the javelin, the Nandaka, and every other weapon, all shining with effulgence, and upraised for striking. And from his eyes and nose and ears and every part of his body, issued fierce sparks of fire mixed with smoke. And from the pores of his body issued sparks of fire like unto the rays of the sun. And beholding that awful form of the high-souled Kesava, all the kings closed their eyes with affrighted hearts, except Drona, Bhishma, and Vidura, endued with great intelligence, greatly blessed Sanjaya and the Rishis possessed of wealth of asceticism, for the divine Janardana gave unto them this divine sight on the occasion. And beholding in the (Kuru) court that highly wonderful sight, celestial drums beat (in the sky) and a floral shower fell (upon him). And the whole Earth trembled (at the time) and the oceans were agitated. And, O bull of the Bharata's race, all the denizens of the earth were filled with great wonder. Then that tiger among men, that chastiser of foes, withdrew that divine and highly wonderful, and extremely varied and auspicious form. And arm-in-arm with Satyaki on one side and Hridika's son (Kritavarma) on the other, and obtaining permission of the Rishis, the slayer of Madhu went out.
Another theory is that they went into Samadhi (or Turiya I am not sure which) in which they heard the divine truths.
According to my knowledge, even in Christianity, the Divine messenger , archangel, Gabriel was depicted in Human form with wings. So we cannot restrict this depiction to Hinduism alone.
Coming to the question part - "how did the rishis/munis who wrote Vedas/Purans after listening to Ākāśavāni predicted how a particular god would look?", the following are my ideas:
The word Ākāśavāni used by the OP requires clarification. Ākāśavāni means listening to something that came from outside. It should be अंतर्वाणी - antarvani - something that came from within. The sages/rishies realised the Knowledge (Jnana) from the inner voice only.
There was no concept of idol worship in ancient/vedic era of Sanatana Dharma aka Hinduism. The attribution of certain human forms to certain deities is not Vedic. The tradition of idol worship/attribution of human forms might have emanated after Jainism and Buddhism started propagation of their ideas of Salvation.
In the post Vedic era, Ramayana was written. If we omit the interpolations, and stick to the RAMAYANA text proper, we can understand that this EPIC was composed based on Vedic traditions only. And, idol worship was not mentioned in Ramayana.
While roaming in the Forest, Sri Rama along with Sita and Lakshmana, visits the hermitage of Sage Agastya. There it was mentioned that various fire places were fixed for offering oblations to various Gods.
स तत्र ब्रह्मणः स्थानम् अग्नेः स्थानम् तथैव च || ३-१२-१७ विष्णोः स्थानम् महेन्द्रस्य स्थानम् चैव विवस्वतः | सोम स्थानम् भग स्थानम् स्थानम् कौबेरम् एव च || ३-१२-१८ धातुर् विधातुः स्थानम् च वायोः स्थानम् तथैव च | स्थानम् च पाश हस्तस्य वारुणस्य महात्मनः || ३-१२-१९ स्थानम् तथैव गायत्र्या वसूनाम् स्थानम् एव च | स्थानम् च नागराजस्य गरुड स्थानम् एव च || ३-१२-२० कार्तिकेयस्य च स्थानम् धर्म स्थानम् च पश्यति |
Rama entered inside the hermitage and saw therein the sanctus of Brahma, Fire-god, Vishnu, Indra, Vivasvat - the Sun-god, Soma - the Moon-god, Bhaga - one among the twelve Suns, and the sanctusms of Kubera, [Wealth-Management-god, are seen and passed by the three of them, sanctums of Dhaata, Vidhaata - Vedic deities created by Brahma to help Svayambhuu Manu, santucm of Vaayu - the Air-god, and also like that the sanctum of great-soloed VaruNa - the Rain-god who also wields noose, and the sanctum of Gayatri - the presiding deity of gnosis, sanctum of Vasus - eight of them, and the sanctum of cobra's king - aadi sheSa, the divine Thousand-headed serpent that bears this globe on its head, and on which Vishnu reclines, and even the sanctum of GaruDa - the Divine Eagle and the vehicle of Vishnu, and the half brother of aadi sheSa, and the sanctum of Kaartikeya - chief of gods army, second son of Shiva, and the sanctum of Dharma - Dharmaraaja, presiding deity of Virtue-Vice-Time of living beings, in-charge of the hell.
A true rishi is one who is a devotee of lord also. Spiritual path is a heart to heart connection. So the lord reveals himself to his devotee in heart. Citing this verse from Srimad Bhagvatam 10.86.47
हृदिस्थोऽप्यतिदूरस्थ: कर्मविक्षिप्तचेतसाम् । आत्मशक्तिभिरग्राह्योऽप्यन्त्युपेतगुणात्मनाम् ॥ ४७ ॥ But although You reside within the heart, You are very far away from those whose minds are disturbed by their entanglement in material work. Indeed, no one can grasp You by his material powers, for You reveal Yourself only in the hearts of those who have learned to appreciate Your transcendental qualities.
Ramayana revealed into the heart of Valmiki muni and such kind of literature is free from the human defects and is perfect. Vedas are also such an example. it is perfect as it was revealed in the heart of Vyasdev. Thats why you see many things you find now a days going coherent with vedic texts as in astronomy sciences etc and this is just the material part which is presented without any human defects then what to speak of spiritual merits it offers to mankind.