Depending on your PICO question, there are some publication types you can focus on to answer your question:
Most Clinical Questions: Meta-analysis, Systematic Reviews
Therapy: Randomized Control Trials
Diagnosis: Prospective, blind controlled trials compared to a gold standard
Prognosis: Cohort Study, Case Control, Case Series or Case Reports
Prevention: Randomized Control Trials, Cohort Studies, Case Control
Etiology/Harm: Randomized Control Trials, Cohort Study, Case Control
Cost Analysis: Economic Analysis
Where Does the Publication Type show on an indexed article?
When you are determining which category an article falls under, there are some things you should consider. This document with screenshots offer tips for assessing the level of evidence from five different articles in PubMed.