And the answer to this question is, there are no specific credentials for being a medical writer. It isn't easy to know right off the cuff, but there are certain methods that all medical writers should use when writing up a story and indicators that you as a consumer should look for to assure that the information you are getting is accurate and unbiased.
Some medical writers cover every topic from cancer to obesity.
Obviously a reporter who specializes in a specific area of medicine is going to know more about it than a generalist. A good medical journalist should know the basics of study design, but may not be competent to know whether all the proper testing procedures have been used in an experiment. This is why it is important to select articles carefully. One way of insuring good methods is to cover peer-reviewed medical articles. The peer review process looks at the methods and conclusions and then the reviewers decide whether or not the study is publication worthy.