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

The sentence structure gives it away many times.I too admire anyone that can navigate English as a second language.

Nothing worse than an English only speaker criticizing someone whose second or third language is English.

It’s kind of like a Turing test for language....

In terms of detecting that someone is using English as their non-native language, it would also depend on the level of mastery of the “foreign writer” and the educational level of the reader. 

If anyone likes to read, check out Through the Language Glass by Guy Deutscher. He explains how language actually shapes thoughts. An example may be that French contains 7500 different ways to express color. This is an exaggerated number but it proves they can perceive more about something’s beauty because they have a lot more references.  Also German has a lot more direct and absolute terms. While it may seem “harder” it’s beneficial in engineering.  It’s a good read if your into the subject. I thought it was fascinating. His daughter is Alma Deutscher, the violin and piano prodigy. 

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

There is the different spelling of numerous words in American English, secondly, in my learning, American English seems to have a preference for a different (more simple?) sentence structure. 
This also goes for the 'preferred' absents of punctuation marks, other than full stops (i.e. periods :) ! 

This is not negative by any means, and can often contribute to a faster apprehension of what has been said. This also by the shorter sentences. 

However! It also lends itself to saying things that can be equivocal, and in my experience perhaps on purpose - when e.g. reading marketing 'spiel' like IT manuals.

I'd happened to translate numerous into German and often had a tough time to decide which way to say it. German is generally a lot less ambiguous.

Badly misspelled English text (forget 'your' instead 'you're' :) , add some truely - of the wall- grammar, is easy to detect - often quite hard to comprehend as to WHAT on earth exactly was meant. 

M. 🇿🇦