Make sure you can identify the title of the study in question and the journal in which it was published. Don't ever take a blog post or a news story reporting on the results as an accurate representation.
If you can't read the entire study for yourself, use the reputation and the peer review process of the journal in which the study was published to judge how thoroughly the research may have been vetted prior to publication.
Look for follow-ups or critiques to the study that may have been published. See if the research has been reproduced anywhere else or if any similar studies have obtained similar results.
Make sure the study used appropriate controls and statistical methods. Ask yourself: “If an identical study had been run with all samples/groups/etc following the control procedure, would a statistically significant result be obtained the expected percentage of the time?”