maxey pointed out some good aspects. I always believe, if you can speak fluently when you are angry, shouting, arguing, quarrelling, abusing then you have learnt the language well. when we are angry, shouting, arguing, quarrelling, abusing mostly our brains are not working. Its the habit of speaking the language which makes you speak. When you can speak a language perfectly without bothering your brain, just using your habit, you are nothing but a native speaker.

He can speak it!

1. Can you read the front page of a newspaper. Just consider articles in a foreign language about your country. 2. Can you watch action films in the second language and know 90% of what's going on. 3. Can you carry on a non-technical conversation with most native speakers in that language with just a little slang. 4. To be most outrageous, could you be parachuted into the country of that language with a reasonable amount of money and one credit cart and make your back to your native country.

1. To understand jokes based over words' ambiguity or assonances 2. To invent those kind of jokes.

Many people say it is when you can switch from thinking or speaking your natural language to second language at ease. Or when you dream in the second language.