Level 1 - Systematic reviews or meta-analyses of randomized controlled trials. (See pages 582 & 578 of Melnyk text for definitions)
Level 2 - Randomized controlled trial(s). (See page 580-81 of Melnyk text for definitions)
Level 3 - Controlled trial(s) without randomization.
Level 4 - Case-control or cohort studies. (See pages 572 & 573 of Melnyk text for definitions)
Level 5 - Systematic review(s) of descriptive or qualitative studies.
Level 6 - Single descriptive or qualitative study. (See pages 574 & 580 of Melnyk text for definitions)
Level 7 - Expert opinion.
Reference: Melnyk, B.M. & Fineout-Overholt, E. (2011). Evidence-based practice in nursing and healthcare: A guide to best practice. Philadelphia: Lippincott, Williams & Wilkins. See page 12 for LOE Chart.
1) Sometimes it will say right in the article abstract what type of research study/what research methodology was used. Other times, you'll have to go into the full-text article and start reading! This information is often found in the methodology section, but sometimes is mentioned earlier...
2) Most databases (CINAHL, PubMed, etc.) allow you to limit by publication type or research methodology. You can try limiting to RCTs, Systematic Reviews, etc. The below screenshot shows this section in CINAHL.