At my carrier we don't ask the medic if we should divert. Instead we tell the medic how long we have until we land at our planned destination. We find their assessment of time and need is a better way to determine whether a diversion is required or not.
To answer the opening question. You can generally tell if someone is qualified or not, just on face value. In other words, you can tell the dreamers apart. When dealing with 'medicals' the worst thing is actually those passengers who have taken a first aid course who, not realising crew are first aid trained, can be over enthusiastic.