History major here. One way I always checked on certain sources was to look up scholarly journals on Jstor and look at reviews of books from authors within that field of history that you are reading in. The reviews can be helpful in determining what other scholarly authors, generally in that area of study, believe about the authors point of view, and what shortcoming and or strengths come from that particular book. Their reviews can also help identifying why the author wrote the book and what motivations may underlie the writings.

Checking out the sources that the author uses is also a good tip because that may show what exactly the author is looking at to come up the point of view they are trying to get across to the reader.

I haven't read it but from reading reviews it appears this is exceptionally poor. An astonishing misstep by a professor of Cambridge University, who should know better. It is a selective reading of the history to force it to support the author's thesis.

Simms contends that Hitler was not concerned to oppose Communism and Russian Bolsheviks  but capitalism and anglo-american plutocracy. This is like arguing that frogs don't exist because they are really tadpoles. Yes, in Hitler's first political speeches in late 1919 and early 1920 this was true, but over the course of 1920 Hitler pivoted to the east and began to focus on anti-communism and anti-sovietism, which became his main obsession (In terms of foreign policy at least) for the rest of his life.

The development of Hitler's ideology and explanation of how and why he made this "pivot to the east" isexplored with great detail in Thomas Weber's Becoming Hitler, and I would recommend you throw Simms in the bin and read Weber instead if you have any interest in getting an accurate view of the history.

In addition, see this excellent AH post by /u/kieslowskifan in response to a question I had on this subject.

It’s simply a false representation of his true intentions. The evidence chosen is widely accepted, but Simms was very selective with his evidence, and makes lots of almost nostalgic conjecture about Hitler’s motives, which I personally find to be borderline sycophantic. The evidence to support Hitler being a socialist exists, but there is FAR more evidence to prove the opposite.