Honestly, this is one of the major reasons book reviews exist. If a book is more than a year or two old, and is significant in its field, it will have a trail of reviews [edit: many from other academic specialists in the same field]. The more momentous, from big academic or trade presses, may have very prominent ones that discuss historiography and other elements of theory, or bring together two or three books at the same time to talk about them as part of a whole. Choice, NYT, NY Review of Books, History: Reviews of New Books, and the American Historical Review try to be pretty broad (just to give a sampling from the US); specialist journals have their own reviews, which are often deeper and longer. If you are researching a particular topic, you will quickly come to know who is well known (and in what generation) for their work, and you can narrow down the reading fairly quickly. One of the things we must do in working towards our PhDs is create such reading lists for our comprehensive exams (in the US, at least my program), and book reviews are crucial to figuring out who is on what side of what issue. Sometimes you even find out a little about personal disputes and academic genealogy (who trained whom) in the process, which is pretty enlightening if you're going into our professional briar patch.

If you just want to find a good book on a particular topic, though, the reviews should tell you a great deal. They don't always agree with one another, but you can aggregate them and go in with your eyes open. The other advantage to reviews is that they're fresher than the book they're discussing, so if new work has come out, they'll often mention it too. (Sometimes it's their own work!)

If books on major historical events do disagree, it's usually on extremely minor points, that would only be of interest to professors or PHD candidates. The best books will be peer reviews and will find a consensus among the most respected Historians.

Books like Bill o Reily's 'Killing Lincoln' will be rubbished by the most respected Historians and rightly so. However books likes Jared Diamond's Guns Germs and Steel drew applause when first published but were ultimately rejected because of the wide brush strokes Diamond used in painting his picture of 13000 years of history.

What I'm trying to say is that unless your a professor of history or a PHD candidate then a quick review of the people who have peer reviewed the book should be enough to ensure it's accuracy. Also I believe (here in Europe anyway) most academic Historians would argue over their small differences in Journals rather than books.