Is easy to know when a non english speaker is writting?

Yes. But I really enjoy what one of the above posters refers to as offbeat syntax. A few years ago, I was on a music forum and a French lady reacted to a particular new record by saying "I'm on the ninth cloud." Turns out she was saying the equivalent of "Cloud 9." I enjoyed that expression so much that I drop it on people every once in a while. The blank stares are hilarious.

Yes its easy to identify a non-native speaker unless of course they speak and write perfect English.  I know a woman from Belgium who speaks perfect English with no accent at all.  But she's the only European I've ever known who did not have any accent.  

 My wife has a niece who was born in Deutschland. Came to America at age 10. Speaks  perfect English no accent.  Asked my wife who was also was born there how is her German. She said perfect no German accent. Cannot tell the difference from adult Germans. My wife came to America at age 17 and has a mild  English accent. Thought is was interesting. I think at a  certain age one can do both perfectly. 

Well, in my experience, English is not quite the - common- language shared by many other than American English speakers.

English is the international language of air traffic control, so it is a "common" language for specific uses.   

As for correct English usage, I've come to the conclusion that few people on the Internet care about spelling, grammar, or punctuation.  However, there is a difference between American punctuation and English punctuation that shows up mostly in the use of quotation marks - if people even use them.  

There are specific word choices that can give away the nationality of the writer, such as "whilst," a word an English citizen would use, as opposed to "while" which is generally what an American would use.

Then you find bad spelling such as "alot," which is not an English word, while "allot" does not mean "a lot."   Then you get to the homophones like "brake," and "break" and people often get them confused.

Another category is the total misuse of a word.   My current favorite being "fitment."  Fitment is now frequently used by people to mean the precision of fit between two parts, when in fact, a "fitment" is a piece of furniture.  The word has nothing to do with how precise the relationship is between two parts.

The last is the total desecration of English from text messages.  "ill," no longer means feeling sick, but "'I'll," or "I will."   "U" is the substitute for "you."  This is somewhat understandable with a character-limited format like Twitter, but continuing to use the abbreviations in a character-unlimited response only demonstrates the ignorance or apathy of the person making the post. 

But if you want a true demonstration of a non-native English speaker, it comes in the from of word relationships like,  "...so Please i'll appreciate it if you can withdraw the Ad from Craigslist so that I can be rest assured that I’m in the hand of the item..."

Yes, "...in the hand of the item..." from a Craigslist scam email.  No reason to feel the least bit suspicious, given the bizarre syntax and other English errors.

It's like in the original movie, The Taking of Pehlam, 123, when the villain, played by Robert Shaw (an English mercenary) yells
"Hands in evidence",
 only to be greeted with blank stares and has to say, in disgust,
 "Show me your hands."

All the best,
Nonoise