Properly evaluating the factual content of a biography requires a substantial knowledge of the subject. Speaking for my own field of studies, when we look at books like L. Sprague de Camp's Lovecraft: A Biography (1975) versus S. T. Joshi's I Am Providence: The Life and Times of H. P. Lovecraft (2013), Joshi's biography is longer, contains more detail, a larger bibliography, a more up to date bibliography, better citation of the original sources, and is generally considered to be much more extensive and accurate than de Camp's attempt. That's what a couple more decades of dedicated scholarship gives you, and it is actually the fully an unexpurgated and updated version of his 1996 biography: H. P. Lovecraft: A Life.

But a biography is not just a dry listing of facts, as you pointed out. There is also the interpretation and presentation of the evidence, the narrative spun out of that, and quite often some kind of personal opinion or moral judgment of the individual, their actions, their character. In this regard, both de Camp and Joshi have their personal "takes" on Lovecraft. De Camp has been criticized for being too critical, emphasizing Lovecraft's racism and failings creatively, commercially, and as a husband, and with a great deal of psychologizing. Joshi is more restrained in that regard; he certainly admits to Lovecraft's flaws and errors in life, but doesn't feel the need to beat him up about it or try to assign his problems to various neuroses.

In that regard, every biography comes with an element of interpretation, that only intimate knowledge of the subject can help decipher. Except, of course, when the writer of the biography ventures into the waters of pure fable and bullshit.

Example in this case: The Dream World of H. P. Lovecraft: His Life, His Demons, His Universe (2010) by Donald Tyson. This was written as part of a series of pop-occult books for Llewellyn Publications, and is aimed at that market. So while there is genuine biographical material here (mostly culled from Joshi), there is also a great deal of utter crap dealing with Lovecraft as a subconscious mystic who was prone to shamanistic states and channeling occult concepts. You would think that kind of thing the stuff of fiction (and you'd be correct), but the presence of it in "non-fiction" is sadly somewhat widespread; Paul Roland in The Curious Case of H. P. Lovecraft (2014) is another one trying to write a popular mass-market bio of Lovecraft (mostly ripped from Joshi) who can't stop himself from dipping into occult tendencies and mentions of elementals.

At this point, it's worth bringing up autobiography; an individual's own story in their own words is extremely valuable - and so it is in the case of Lord of a Visible World: An Autobiography in Letters (2019) compiled by S. T. Joshi and David E. Schultz from Lovecraft's voluminous correspondence. However, there's a trap there - a double trap in this case. For one, just because it is the individual's own words does not guarantee either accuracy or freedom from bias; many people either do not know the whole truth, freely omit unpleasant details, and shape the narrative to present their life story in the way they wish it to be read. And, if that material is edited, as is the case here, you also have to take into consideration the biases of the editors.

Which is a long way to say...sometimes you can't evaluate a source fully until you know enough about the source to grapple with the primary and secondary sources it cites. If you're new to the subject, start by looking at reviews for the book, especially in academic or scholarly journals; see who cites the book (Google Scholar is good for that), what work the author(s) have done on the subject, whether the publisher is reputable or not. If everything looks solid, you can have faith that the book probably isn't complete pablum or bunk, but even then...a biography is usually the start of an understanding toward a subject, not an end into itself.