When evaluating, look at the study's research methodology.
Studies are assigned levels of evidence based on their methodology. The evidence pyramid is an easy way to visualize this hierarchy of evidence.
At the top of the pyramid is filtered evidence including systematic reviews, meta-analyses, and critical appraisals. These studies evaluate and synthesize the literature. The top of the pyramid represents the strongest evidence.
At the base of the pyramid is unfiltered evidence including randomized controlled trials, cohort studies and case reports. These are individual reports and studies, also known as the primary literature.
You should seek the highest level of evidence available, but remember that evidence at the top of the pyramid might not exist for your particular research question. If that is the case, you'll need to move down the pyramid to find the strongest evidence that addresses your question.