The two main ways used to evaluate the impact of individual articles center around:
An article's accumulated citations can be used to measure the degree to which the academic research community has been influenced by, or reacted to a publication. It is sometimes difficult to compare citation counts between disciplines as some disciplines cite highly while others don't. The SJR metric in Scopus looks at both the quality of the citing journal and the citations and is a normalised metric as it allows comparison between disciplines.
One thing to keep in mind when looking at an article's citation count is that the number of citations will vary across different databases as each database is drawing citation information from its own different pool of titles.
Alternative metrics track social media buzz, usage, downloads etc and provide another way for researchers to identify how other researchers and readers feel about their article. More information is available here...