How do you know someone is a member of MENSA? They tell you.

If someone needs to tell you how intelligent they are, then they probably don't fulfil my criteria for intelligence.

The people I look up to as being the most intelligent people I know do have some commonalities.

They are interested in many subjects
They are unendingly curious
They believe in lifelong learning
They will listen and ask questions, but they will also share their knowledge in a non-patronising way. They want to learn from other people
You are likely to find many books on different subjects in their homes.
Intelligent people will normally speak and write well, coherently and descriptively.
The intelligent people I know don't watch much TV (although some will not miss a sports match)
They are constantly trying to solve problems, not necessarily mathematical problems, but everyday problems, using logic and their own experiences.
The intelligent folk I know have a wonderful, witty and sharp sense of humour.
Overall, I consider intelligent people to have an excellent general knowledge and who are up to date with current affairs and research.
That is how I spot an intelligent person. Spend a few minutes talking to them and you will very quickly know whether you think they are intelligent.