Number of citations per paragraph is a pretty good indicator in my experience. Typically this is high (close to 1) for good text books in psychology and low (1

Number of citations per paragraph is a pretty good indicator in my experience. Typically this is high (close to 1) for good text books in psychology and low (1 per page or less) in bad ones. Citations isn't everything though, many authors misrepresent the content to fit the narrative and in some areas it's not as important. For instance in math, arguments can be made based on very few axioms and you can evaluate the correctness yourself. In general it helps to have a skeptical mindset, try to figure out the intent of the author by how they treat facts, do they present inconsistent arguments or gloss over relevant fallacies in the claims? Then their intent is probably more about presenting a good argument than correct representation of facts. Another thing to be watchful for is claims that "make sense" or are "obvious", it's easy to fall in the trap of ignoring conflicting evidence when the theory sounds good.