You can use Google Scholar and Web of Science to look up how many times a particular work has been cited. There is no specific number of citations that makes a work landmark. Instead, we are looking for articles that have been cited much more often than other articles in that discipline. Do a few searches to get a sense for how many times other studies of a similar age are cited in your research area.
Google Scholar
In Google Scholar, search for the title of the work inside quotation marks. Under the search result, look for the Cited by link. This will show you how many times a source has been cited:

Web of Science
In Web of Science, select Cited Reference Search from the search options to see how many times a specific source has been cited:

Fill out the search fields with the author, publication date, cited title, and so on. You might see multiple results for a given work:

Select all that apply and click Finish Search. You'll then receive a list of all the documents that have cited a particular publication and can choose to Analyze the results or sort by Times Cited.