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.
the teaching of a perfectly enlightened being would be always in line with the three marks of existence, the four noble truths and the noble eightfold path. in other words, an arahant or a buddha would be completely and permanently objective and selfless and with no other interest than what is conducive to suffering (dukkha), what is the common cause of all suffering (dukkha samudaya), what is the absence of suffering (dukkha nirodha) and what is the method for attaining the absence of suffering (dukkha nirodha gamini patipada).
The Dhamma cannot be discovered by a new Buddha again while the Pali Suttas remain existing in the world & known to humanity. The Suttas (MN 115) say there can only be one Buddha in a world system. A new Buddha can only arise when the Dhamma has been undiscovered or has been lost. The Suttas say:
Mendicants, whether Realized Ones arise or not, this law of nature persists, this regularity of natural principles, this invariance of natural principles... A Realized One understands this and comprehends it, then he explains, teaches, asserts, establishes, clarifies, analyzes and reveals it...
Since most Buddhists, let alone most people, have not understood the Pali Suttas over the last 2,600 years; it is unlikely if a future Buddha were to arise, most people would acknowledge him as perfectly enlightened. Instead, most Buddhists & most people would regard fakes as perfectly enlightened. This appears why most Buddhist take refuge in all of the myriad gurus, commentators & scholars.