There are now several methods for analyzing citation metrics to identify leading journals in various disciplines. There are also limitations to using these methods (see Northwestern University Library, DataBank: Article Level Metrics & Citation Analysis).
Google Scholar Metrics (free) : uses the h-5 index, or h-index based on the last 5 years, to identify highly cited journals across disciplines as well as in individual subject areas.
Journal Citation Reports (JCR – subscription required): provides the journal impact factor, a standard metric based on the number of citations to journals indexed by Web of Science. The Nimitz Library does not currently subscribe to JCR or the Web of Science but they are available at other area libraries.
CiteScore (free): a recent competitor to the journal impact factor that provides a similar ratio of current citation compared ot total number of articles published in the journal during the previous 3 years rather than 2. CiteScore is freely available and based on journals indexed by Scopus, an Elsevier database.
American Journal of Political Science. You can also contact the journal to inquire about the impact factor.: many journals provide the impact factor on the journal website, e.g.,
The h-index calculates the highest h number of articles that are cited h times, an indication of productivity and citation impact of a journal.
Example: a journal with a h-5 index of 50 has published 50 articles that have been cited at least 50 times within the last 5 years.