I think one part of demonstrating expertise in an area is the ability to easily and simply educate others ignorant of that area in its basic and sometimes more advanced facets. There are some extremely intelligent people out there doing good work themselves, but who have terrible "bedside" manner, i.e., the ability to explain their topic and demonstrate their expertise to those upon whom they intend to "inflict" it. That communication is where the trust forms and where the "ignorant" become more educated and thus more trusting of the "expert".

But beware the phenomenon of "Those who can't do, teach" and its corrolary "Those who can't teach, do". There are many instances where the best teachers are not necessarily the best practitioners (I would not let the chief surgeon where I trained operate on my dog let alone myself or my family, but he developed and provided a phenomenal teaching program). And as I stated previously often the best practitioners are out in the real world banging out the cases (to the great benefit of the many patients they see), but when put in a position where they are expected to slow down and explain what they are doing and why they become, well discombobulated------learning from these folks is best done by observation without intrusion, and occasional questioning of purpose of technical aspects of their procedures.

I hold no truck with PhDs (or MDs or other similar educational equivalents), as I owe much of my 14 years of post-high school education to them, although I will say that there were many whose "pearls" were taken with a large grain of salt, especially when they could not communicate with their students, or otherwise demonstrated some connection. And how do you put faith or trust in the teachings of someone who shows up to teach an advanced class everyday in a cardigan sweater (in either maroon or navy blue) which is mis-buttoned by at least one button, and oftentimes by several (one time 6 buttons off on a 7 button sweater). Others who grabbed students' interests and held them and nourished them could be equally eccentric but at least make the connections and trust necessary to make what they taught have more real meaning and value (a color blind genetics professor who was also an amateur painter--yes I said color blind---showing up in the dead of winter in paint stained shorts and shortsleeved shirt with the neck of a bottle of Gilbey's Gin sticking out of his back pocket-------I chose this guy as my pre-med advisor after dumping the turd I was assigned). Many with dedication even continued to hold classes in their off campus homes during a three week professors' strike in order to honor both their commitment to the strike as well as their commitment to the education of the students in whose hands they were entrusted.

Credentials are fine but they only take you so far, with the demonstration of the makeup of those credentials, and personal interaction and interpersonal development of a relationship needed to develop the true trust needed in an expert IMHO. Sometimes we cannot for whatever reason acquire some or all of those pieces and are left making that decision of "expertise" based on what pieces we can put together, a less than ideal situation IMHO.