The best known article level metric is the number of times an article has been cited. You can find these article level metrics in most databases now.
*But* these citation counts are only based on cites from journals indexed in that database.
So, it is best to look in several different databases including some with large coverage such as Web of Science or Scopus.
For suggestions of databases in your subject area see our Library Research Guides.
Basic Instructions:
PLoS is the publisher that has really pioneered article-level metrics. Articles published in PLoS journals have these metrics clearly posted: