Well, the question you ask will procure the same answers when asking about historiography.

Simply put, a biography defendable according to scholarly standards (“academic rigour”) employs effectively the very same methods of heuristic research, source criticism and methodical interpretation as scholarly historiography does.

Therefore, we deal with the same problem: how to evaluate the potential quality of a book when we have no idea?

There are some guidelines:

Check the publisher’s overall track record. For example, is it an academic publishing house, like a University Press (or a major academic house like Routledge, Brill, etc.)? Or is it a publisher specializing in a certain field or genre?

Who is the author? Is it a journalist whose books are all over the place in their topics, or something with an expertise in a relevant field? (For example, I’d not even bother with a biography of a Japanese person written by a person who isn’t evidently fluent in Japanese and displays some familiarity with the culture and history.) Is the author following certain standards of argumentation, and does he even provide arguments in the first place? Is there a bibliography, are there foot- or endnotes pointing to the evidence used?

What do reviews, especially in academic journals, say about the book? Is it well-received by scholars?

Of course, I don’t know what exactly you mean with “The real deal,” but when it comes to biography, I can say the following:

Since humans are per definition multi-dimensional personalities, any biography will be focused on a number of main aspects that the biographer considers “important,” and not just describe everything we can possibly know about the person indiscriminately. The biographer has no other choice if he wants to produce a work which makes sense and thus produces knowledge. A good century ago, Virginia Woolf expressed this fundamental conundrum of the biographer succinctly in the phrase: “A biography is considered complete if it merely accounts for six or seven selves whereas a person may well have as many thousand.”

This means that a “definitive” biography cannot exist (in the same way as a definitive historiographical text of something cannot exist).

Instead, a biography is a portrait of a person, which is only founded on the interpretation of sources: nothing is invented outright! As long as this portrait is constructed methodically, then it may result in the production of biographical knowledge. But like a portrait, it can’t replace the real person behind it: there always can be, and often will be, several biographies which are all equally “The real deal.”

[edit:]This is further complicated by the fact that under the umbrella of biography, we can find a plethora archetypes: is it to be a study of character, a study of a person in interaction with their social and cultural circumstances (very typical of historical biographies), a study which uses its protagonist to shed light on something more general, such as issues of race, gender, class, and the like? It may also serve to interpret a person's legacy, such as is common with biographies of scholars, artists, or writers: these would discuss the life in relationship to the work. All of these are valid approaches, and its not possible to do all of them at the same time. Hence, there are many valid biographies, and one might say that there is even a need for them. And, of course, this also means that you might want to think about what you're trying to get out of a biography. [/edit end]

But a "biography" which just tells you straight-up “How it was” without leaving a margin of doubt, without any reflection of the author's side, without making its methods and its sources transparent at all—such a work is likely endeavoring to construct a heroic myth, and shouldn't be called a biography (but rather a hagiography).

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The above-mentioned applies to online-resources, to some extent: a YouTube video which remains silent on its sources may unexpectedly be good, or it may be nonsensical fiction. Someone who actually put in effort into researching will have no problems with telling their audience what they used. Even documentaries are also problematic: not even the featuring of academic experts on the topic are a guarantee it will be good, since its not exactly impossible that their statements will just be cut out of the full interview so that they fit into the narrative the documentary writer (who usually is not an expert) wants to present.

In short, there unfortunately is no surefire way to be certain about the quality of anything without any prior knowledge, but there are some means to figure out if a book might likely be worth a read or not.

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(Some references for the writing and evaluation of biography are:

Caine, Barbara. Biography and History. Theory and History. Second Edition ed. London: Red Globe Press, 2019. First Edition 2010.

Edel, Leon. Writing Lives: Principia Biographica. New York and London: W. W. Norton & Company, 1984.

Etzemüller, Thomas. Biographien: Lesen - erforschen - erzählen. Historische Einführungen. Frankfurt/New York: Campus Verlag, 2012.

Fetz, Bernhard, and Hannes Schweiger, eds. Die Biographie: Zur Grundlegung ihrer Theorie. Berlin, New York: De Gruyter, 2009.

Possing, Birgitte. Understanding Biographies: On Biographies in History and Stories in Biography. Translated by Gaye Gynoch. University of Southern Denmark Studies in History and Social Sciences. Odense: University Press of Southern Denmark, 2017.

Of these, Edel's book is a splendid read: the man received the Pulitzer Prize for a reason. I also took the Woolfe-quote from his book, pg. 132. His annotations are unfortunately a bit shoddy, like he himself laments, but the original should be from her novel Orlando, likely pg. 189/190.)