For most cases, a respectable editorial company among scholars is preferable (like De Gruyter, for example), universities' presses and looking for the books' reviews on special journals (available in places like jstor) usually filters a lot. There are gems outside of those circles with interesting perspectives, but for academic level of research, the aforementioned circles tend to be the safer option. Searching for the authors' CV and their other publications also helps. Now, internal critique is another story, as there is a history of the historiography of any historical subject and not every professor teaches their students about it nor mention it. One way to tell of what is conjecture or not is to see how based on primary sources the authors are, if there is a stretchy conclusion or with anachronisms that the primary sources themselves never stated or mentioned anything that resemble it. Depending on the area, many conjectures are reasonable and believable, but others are complicated - so this is why is important to check the primary sources and informed reviews.

If you have a book about Talleyrand that was written by a scholar, look at their bibliography; usually they have to separate the primary sources and state their location - you will be able to see if it came from a book that compiles letters/manuscripts (very common in many areas) or the manuscripts themselves and their location. If the book of primary sources is from the nineteenth century or early 1900s, there is a great chance that you will find it available online. Some institutions also make available digitised manuscripts to the general public for free, so it may be worth to search for the manuscripts as well. Considering that Talleyrand was french, my suggestion is to also look for books about him in french, as there is a bigger chance of having more research about him from local historians. Talleyrand and his period are not my speciality, but I hope my answer can help you.