[gremlinunderway]

i actually don't think you can tell immediately at all. Being an expert at a thing and also being a good communicator or teacher of that thing is absolutely not the same thing.

One can be an expert on a topic and produce research, or tools, or predictions, but could be terrible at pedagogy or communicating those thoughts to those outside of the field.

People are really quick to forget that there is an entire field of study dedicated to teaching people, and the reason there is this field of study is because its a lot more complex then people would like to think. Lots of smart people fall for this trap constantly thinking that "well I'm smart, so what I say must be smart", but then completely missing that their communication or output is off the mark entirely for an intended audience. Those people tend to then just shrug it off as "well they're stupid" (on the harsh end) or "well they don't know enough about X to understand this..." (on the less harsh end) rather than ever considering "could I have delivered or explained this better"?

It is very difficult to explain complex things in simple terms. It's probably the most complex thing humans cognitively do (synthesis vs analysis). Synthesizing a complex and very dynamic system or topic into analogies that are both (a) accurate models/shapes of the original and (b) understandable for non-experts is its own challenge separate from even understanding the topic itself at an expert level.