From in my personal experience there are several things to look at:

1. Who is the author? You can check the back of the book for a brief biography, or look them up online. If they are a professor or leading scholar in the field, they should have some web presence - such as a university page if they teach, a wikipedia page if they are a famous academic, or perhaps a website where they at least list their credentials.

In my experience, pop historians might have a bachelor's degree in history or an unrelated field, whereas more authoritative books tend to be written by those with a higher degree or significant experience in their subject area.

2. What type of publisher ran the book? Basically, was it a university press that publishes academic work or was it a publisher known for fiction or "pop" non-fiction? See what else that publisher has done.

3. Who specifically published the book? Within academic circles, some publishers are considered more notable than others - think Harvard, Stanford, Oxford University Presses, etc. That's not to say Podunk University isn't credible - just as a general rule the top quality work tends to be found from the most prestigious publishers.

I've worked with professors who first sort literature that they plan on using for their books-in-progress/research based on the prestige and recognition of the publisher - for example, a book published by Harvard was initially given a higher priority than a book published by Temple University. That's not because Temple can't publish good books as well, it's just academic triage of a sort, when presented with a huge volume of existing literature.

4. What are the book's sources? Does the author offer footnotes, or citations at the end of the book? How much does the author engage with the established literature in the field, specifically what already relates to the topic at hand? This point is especially important!

These are just a couple things I have in mind when I approach new material.

Now, you ask if pop history is mutually exclusive with scholarly history? From what I gather the definition of popular history seems to be a genre which "aims at a wide readership, and usually emphasizes narrative, personality, and vivid detail over scholarly analysis" (taken from wikipedia). This would seem to make it more or less mutually exclusive with academic sources, which value scholarly analysis and would not value narrative, personality, or vivid detail.