You can generally tell the people who *really* know their stuff because they'll talk about it a transparent way, using ordinary language and not afraid that you'll see flaws in it, nor upset if you do

Perhaps they really know their stuff because they've had this attitude since long before they were really good at what they do.

As BUU points out, there are reasons to use advanced technical language (if you can call it that) but often people fail to consider the knowledge level of the person they're speaking with. Being able to talk about a subject in terms anyone can comprehend is a very valuable skill and definately shows a strong understanding of the subject.