True incarnations never self-proclaim; they are known only through the symptoms described in revealed scriptures.Listen — Srila Prabhupada Uvaca
Prabhupāda: sarvajña munira vākya-śāstra-'paramāṇa' āmā-sabā jīvera haya śāstra-dvārā 'jñāna' [Cc Madhya 20.353] [The Vedic literatures composed by the omniscient Mahāmuni Vyāsadeva are evidence of all spiritual existence. Only through these revealed scriptures can all conditioned souls attain knowledge.] We should always think that we are in the modes of ignorance. We are just trying to make progress from ignorance to the goodness, and then transcend. This is the process of spiritual realization.
Nobody should think that "We are perfect." We cannot be. God is... Only God is perfect, and we are all imperfect. Even our so-called liberated stage, we are still imperfect.
Therefore one has to take shelter of authority because, constitutionally, we are imperfect. Lord Caitanya says, āmā-sabā jīvera haya śāstra-dvārā 'jñāna'. So therefore, for real knowledge, we have to consult the scriptures, śāstra. Sādhu-śāstra-guru.
Sādhu means pious, religious, honest person. Sādhu. Whose character is spotless, he's called sādhu. Śāstra means scripture, and guru, guru means spiritual master. They are on the equal level.
Why? Because the medium is scripture. Guru is considered to be liberated because he follows the scripture. Sādhu is considered to be honest and saintly because he follows scripture.
Sādhu-śāstra-guru-vākya. Nobody can become a sādhu if he does not accept the principles of scripture. Nobody can be accepted as guru, or spiritual master, if he does not follow the principles of scripture. This is the test. tad-vijñānārthaṃ sa gurum eva abhigacchet samit-pāṇiḥ śrotriyaṃ brahma-niṣṭham [Muṇòaka Upaniṣad 1.2.12] [To understand these things properly, one must humbly approach, with firewood in hand, a spiritual master who is learned in the Vedas and firmly devoted to the Absolute Truth.] Śrotriyam, śrotriyam means one who has accepted the Vedic literature, the śāstra, scripture, as the guidance.
He can be... Not a extravagant upstart, he makes some group and religious principle of his own and become a guru. No, he's not guru... So Lord Caitanya advises... Sanātana Gosvāmī's enquiry is how to know that he..., here is a avatāra.