There is no hard and fast way to determine if a secondary source is reliable or not, but there are a couple of things you can check for to assess the work.

1. Reviews - as you mention, most reputable scholarly works will have reviews published in various historical journals. If the work was published in the last two years, it may have fewer or no reviews, so this one mainly applies to works published before 2015.

2. Sources - Does the author use primary sources and cite them regularly? There are some series (Penguin history of England, Hill & Wang series on American History) that don't cite their sources, but all have bibliographic essays that indicate their sources. Check to see if they are just using published records, or if they are going to various archives. Works with lots of archival research tend to be better suited to researching in undergrad. Granted, this will not always be the case, and many synthetic works are excellent, yet cite little to no archival material.

3. Publisher - Again, this has it's exceptions, but one of the better ways to root out good works is to see what publishing house accepted them (or what series they are printed in). Certain presses will specialize in certain fields, like the University of Wisconsin publishes excellent works in African History, or the University of North Carolina on early American history. For your era, you would want to look at Oxford & Cambridge as the leading two presses. Cambridge's series Cambridge Studies in Early Modern British History is a fantastic series, and are often great examples of solid Ph.D. writing (as many books printed by that series are based on dissertation work).

4. Do other works cite it? For works older than 5 or so years, one of the best ways to tell if a work is influential is to see if other works published recently have cited it. More citations typically means it's a better work. Obviously if the work is being cited for it's flaws or misinterpretations, then that's a solid indicator that the work is a poorer selection.

5. Author's reputation and career - It's almost always worth googling the author of any book, especially when checking a works veracity. Does the author work for a university? Are they a mainly a journalist? What types of works have they published in the past? All these can help narrow down whose works are geared more towards entertainment or a popular audience, versus a scholarly audience.

Just to note; there are exceptions to all of the above rules, but when first starting out, they can be helpful in culling down massive lists of books into a few key works that can help you get started.

I have not encountered God's Traitors before. By all my criteria above, it looks like a reliable work (I did not see any reviews of it, and though the author appears to be a writer by trade, she has published in Tudor History before). However, it certainly appears more geared to a popular audience - her writing style and many of the assumptions she makes (or rather, doesn't make) indicate an audience that is less familiar with the source material.

What are you looking to research? You have a time and place, which is good, but there are hundreds of topics to pick from in sixteenth century England. Are you looking at Catholicism in Elizabethan England? The Reformation in England more generally? I'd be glad to help point you towards some works I am more familiar with.