Preface: I actually have 2 answers to this question, both tackle different aspect of the question.....

How do you quickly identify with near certainty the smartest (most intelligent) person in a room of people?

it may be hard, because intelligent people seldom give out the signs of their intelligence. they don't "flaunt" their smart.. and intelligence isn't immediately obvious.

However, there is another way you can spot the smartest person in the room. 

LOOK FOR THE MOST POWERFUL PERSON IN THE ROOM.

They may not be the boss by title, but that person is certainly the alpha. Feel the power dynamic of the room. Look for him/her, and ASK them something. Ask them a question that he is able to answer but not something obvious or easy to answer.

Pay attention to how he/she answers you. If the person did not outright seek confirmation of another person (aka.. "Is that right? Sam?"), you would see the obvious body language that conveys seeking of conformation.

Identify the smartest people in a group by identifying the most powerful people in the room. He/She would point you the smartest person.