In CINAHL's Advanced Search you'll find the checkbox option for Research Article. You'll also see a Journal Subset option. Use Journal Subset to select Nursing Journals, as nurses are most likely to publish in nursing journals.
As the image above states, ideally you won't use the First Author is Nurse or Any Author is Nurse checkboxes, as they will miss many nursing research articles.
Once you're at your CINAHL results set, often the article title will be a good indicator that a study was performed, as will the abstract that is often available in the full record. After clicking on the Find It link for full text options, review the article webpage or PDF version of the article, as those will often have the author's credentials, and the full text will confirm if a study has been performed (research) as opposed to an article that is an overview, CEU, case report, editorial, etc.