Don't believe what you see, heard or read. Ask questions. For example, if you hear that 4 out of 5 dentist's approve of some toothbrush there are some questions to ask. What was the actual number of dentists surveyed (if it is a low number, it can not be trusted)? Why did the 5th dentist not approve? Why did the 4 approve? What did they like about it? and the questions could go on.
We have been told "Butter is bad", "Butter is good" and many other scientific flip flops in history. We need to ask more questions.