CC Madhya 20.354

avatāra nāhi kahe — ‘āmi avatāra’
muni saba jāni’ kare lakṣaṇa-vicāra
Word for word: 
avatāra — the actual incarnation of Godhead; nāhi — never; kahe — says; āmi avatāra — I am an incarnation; muni — the great sage Mahāmuni Vyāsadeva; saba jāni’ — knowing all (past, present and future); kare lakṣaṇa-vicāra — describes the symptoms of the avatāras.
Translation: 
“An actual incarnation of God never says ‘I am God’ or ‘I am an incarnation of God.’ The great sage Vyāsadeva, knowing all, has already recorded the characteristics of the avatāras in the śāstras.
Purport: 

In this verse it is clearly stated that a real incarnation of God never claims to be a real incarnation. According to the symptoms described in the śāstra, one can understand who is an avatāra and who is not.