Secondary sources can be books or articles found through the library catalog or databases. Here are some things to check for to be sure you are getting quality material:
What is the publication date?
Information may be dated by the time it is published as new information can come to light or new interpretations can change the usefulness of the piece.
Be careful with publication dates as reprints may feature the reprinted date prominently and the actual date could be much older.
Is this a revised edition? What revisions were made as simply correcting grammatical errors is not a substantial revision.
Who wrote it?
You can look for degrees, institutional affiliation, etc. on the internet but be sure you are looking at the correct person.
If the piece has two or more authors, they act as peer reviewers for each other (even if it is a book) and catch mistakes, etc. to improve the writing and research.
Who published the item?
University presses are some of the best publishers as they have the institutional reputation to protect.
There are high quality publishers that are not university-affiliated so do not dismiss them out of hand.
There are pay-to-publish predatory companies to watch out for. Check with your professor or librarian when you are not sure.
Self-published is not acceptable for secondary research.
Works published as textbooks are not generally considered scholarly, especially if written for the K-12 level.
Is there an editor (book) or peer-review (article?)
Editors do more than check grammar and writing style. They generally have a say in who contributes to the work and will only want the best scholars.
Peer-reviewed articles meant the manuscript was submitted to one or more recognized experts on the topic for comments and recommendations regarding whether to publish or not. This adds to the time before publication but will ensure a better quality resource.
Bibliography or notes
What sort of resources are cited?
Are there statements made in the text that should be supported by a cited source that are not?