How do we know what to believe?

1) You can become an expert in evidence-based dentistry. And once an "expert" you will likely be a bit disappointed regarding how seldom the precise question you want answered will be addressed by high level studies, e.g., blinded, controlled trials.
2) Even when you find high-level science that goes to your clinical question, you must remember that such studies compare average outcomes. While averages are useful, remember that no patient is "average" in every way.
3) You can subscribe to the two EB journals in dentistry, namely the Journal of Evidence Based Dentistry and the Journal of Evidence Based Dental Practice where "experts" review what's been in the literature and can give you insights into how good the science is. Unfortunately, neither of these resources is infallible.
4) One can dutifully dig thru the Cochrane Reviews, now available to you on-line as part of your ADA membership. Cochrane Reviews are the gold standard, but not every review is perfect.
5) One can tune-into the ADA's Evidence-based Center for tips and reviews of systematic reviews -- the most thoughtful opinions I've seen anywhere.

Bottom line is that evidence exists to lead us to the truth. Yet, evidence may be incomplete (often is), inappropriate, out-of-date or otherwise limited in its applicability to the clinical question we seek answered. Nevertheless, EB efforts are usually better than just guessing or relying on what one learned in dental school or residency and perhaps better than the latest "nugget" from an über-hyped CE speaker. The fact that there are unlimited permutations regarding how existing evidence applies to each and every patient at each and every time in his life, is why the ADA's definition of EB dentistry goes along the lines of the "judicious use" of the "best available evidence" in the context of a patient's needs and desires and the practitioner's skill and judgment.

IOW, EB dentistry is one more step along a continuum seeking clinical excellence.