Because JSTOR and Project MUSE are collections of academic journals, the articles you find in these databases will generally be scholarly. If you'd like to verify that an article is peer-reviewed, use Ulrich's Periodicals Directory (the call number is 070.5 UL7 in our reference books). Peer-reviewed journals will be listed there as Refereed. If you're still not sure, please ask a reference librarian for help!
Note: these databases include book reviews, introductory material for journal issues written by editors, and similar materials. These are not considered peer-reviewed articles.