There are at least two ways to find the answer to this.
1. Find the journal's home page and look for a page entitled "information for authors" or something similar. For example for Nature the route is: For authors/Getting published in Nature/Manuscript FormattingGuide/5.4 References.
2. Login to your RefWorks account, select a few citations, then go through the steps to Create a Bibliography...Preview Output Style...open the drop menu box and scroll through the thousands of options till you find the journal.
2a. Many of the citation management softwares will also do this for you.