This has always been a point of mixed amazement and befuddlement for me (a layman) when discussing what I've read with my friends who have advanced degrees in history (or some such similar subject). They'll often comment that a book was "well-researched" or has "terrible scholarship" or a similar remark.
I've never wholly understood how they reach these conclusions and I'd like to know how one does this so I can start making these investigations and assessments myself.
There have been previous threads on secondary sources and how to best judge them in this subreddit. In particular, I'll point you to these two threads:
The top level replies are excellent and should give you an overview on how academics determine if a work is good scholarship or not.
Hope this helps!