There are no "rules of thumb" in the Universe. You need to use your intellect to determine who is who. The most important thing to remember is to pay attention NOT to appearances, but what is BEHIND them.
One day an old man came to a doctor and said:
"Doc, I am 72 and can only make love with my wife once a month, but my neighbour is 93 and he says that he can make it everyday..."
Doctor smiled and answered: "Why don't you say it too??"
What counts most is NOT appearances but what is BEHIND them. Do you remember the chapter "Not what but WHY?"
One of the universal techniques to study ANY unknown system (including someone's intellect) is to "disturb it" and observe how it reacts. Doesn't a child bang a toy to find out what is inside? Don't our scientists bang atoms for the same reason?
Banging someone's intellect can be accomplished by asking "questions" and carefully LISTENING to the response. Usually it is better to start with almost "innocent" rather than provocative questions and focus on listening.