In every field or discipline, there are journals that scholars consider highly prestigious and influential. Professors can easily name the best journals in their fields.
In addition to prestige, which is difficult to measure, journals are judged by a variety of metrics that look at the impact of articles published in that journal. The metrics usually look at how many citations that papers published in the journal receive over time. (Note: These metrics have received criticism because they are easy to game and are not an accurate measure of quality.)
In the library database, Journal Citation Reports, you can search for a specific journal to see its impact factor or view journals ranked by impact factor in a specific subject category. Choose the Select Categories button in the left navigation menu to see the appropriate list of subject categories.
To view journals in a specific subject ranked by impact factor, sort the list by impact factor.
Eigenfactor looks at the impact of journals that are citing each article. On their website, you can search for a specific journal to view their Eigenfactor or search by impact factor category to choose a specific field and view the journals ranked by Eigenfactor. If you want to learn more about how Eigenfactor is calculated, take a look at this article that explains the methodology.
To learn more, see the Impact Factors and Citation Analysis Research Guide.
If you need to figure out whether the specific article or book you're looking at is significant, check out Determining whether a Work is Significant in my Field.