If hypothetically we lived in a time when a Buddha were to arise, how could we acknowledge him as being perfectly enlightened?

Assuming that in a hypothetical multitude of so-called "enlightened masters", a future Buddha would actually arise and we were alive at that time, how would we be able to recognize him? The question could also be valid for acknowledging an Arahant or any enlightened person not necessarily a Samma Sambuddha, but would be much more interesting to answer it in the context of just before a new Sangha is formed, assuming a person declared that the Dhamma has been discovered again.