Web of Science "times cited" count is one simple and popular metric for determining an article or author's impact. You can search for an author, a single article, or a topic and find how many times articles have been cited within the Web of Science databases. Universities often look at this information for hiring, promotion and tenure decisions.
When looking at an author's output or topic, you can change the default sort order of the articles from "Publication Date - newest to oldest" to "Times Cited - highest to lowest." This makes it easy to find some of the most important articles in a topic.
Google Scholar can also be used for citation counts.
There are several problems inherent in using times cited as a measurement of quality: Review articles are cited disproportionately, author's cite themselves to raise their citation counts, articles in certain journals are more likely to be cited than others, citation counts vary greatly across disciplines, and, of course, people may cite an article because they are responding to errors or other problems with the article, not because it represents excellent scholarship.