1. Use Web of Science to find frequently cited works. To the left of the results are Quick Filters, one of which is "Highly Cited Papers." Highly cited papers are those that receive enough citations to place them in the top % of their academic field, based on a "highly cited threshold for the field and publication year." This is a different measure than total number of citations, which may show that a work got a lot of attention, but not necessarily that the work was seminal. One drawback of Web of Science is that they only count citations in journals covered by Web of Science. Indexing of journals in the Sciences is much more extensive than indexing in the Social Sciences and Humanities.
2. Use a database or Google Scholar to find review articles. Review articles provide an analysis of the state of research on a topic. They typically summarize key findings, reference seminal works, and identify research gaps and directions for future research.
3. Check out the literature reviews in dissertations related to your topic.
4. Bibliographies like those that appear in Oxford Bibliographies Online often identify seminal works.
5. The articles in research encyclopedias such as the Oxford Research Encyclopedia of American History often have discussions of the literature that will highlight seminal works.