the most important thing is sourcing, if a book cites credible sources its more likely that the statement is true, in doubt look up the cited sources for yourself

step two is to look how credible the author is, if he/she is shunned by the scientific community he is probably not a good source, but keep in mind that in all sciences there are contrary theories supported by different parts of the scientific community. Thats why its a big difference if the author and his theories are seen as not credible by parts of the community because they may have dissenting theories or if more or less the whole community shuns the author as a fringe scientist

step three is to compare the statement or theory with other books about the topic, if for example 5 well sourced books of credible authors state the same its probably true

but keep in mind that it is hard in historical sciences to have an objective truth, because you can analyze historical events from different perspectives and giving some factors more weighting than others when explaining the reason behind an event.

For Example you can look at the reasons for the WW2 from a political,economical,psychological,cultural or a class perspective, and depending how you think how important each of these factors are in the outbreak of the war you will get another conclusion.

thats why there are still different theories about why the war started

Thank you for this very detailed answer! It is very helpful and very informative! Thank you for your time and help!