You are correct in that it isn't easy to tell what someone is like after a short meeting, but it does require some experience of getting it right and sometimes getting it wrong. I have done both and learnt each time to refine what I'm looking for in a prospective FI.
What I didn't particularily like was being put in a totally artificial situation by someone with less than 1/4 of my experience at the time. Each place does things differently and everyone takes time to adapt to it, but it would help if people were told what is expected. Certainly something I tell all of my FI's. i.e How I expect certain lessons to be taught so that we can keep some 'semblence of standardisation.
I have found very fortunately that most people are very competent and that given guidance even someone seemingly a bit ropey can be brought on leaps and bounds, but the key is personality and how they relate to those around them. That I've found to be far more important than trying to trip someone up by pretending to be a "Bloggs." That works OK if the person testing is an FII or FIE, but not if they are still wet behind the ears themselves, since to be honest you haven't the experience to know what to look for.
Now I do, but 4 years ago? Probably not.