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

Here is a translation of your question...

Welp, self explanatory.

In spaish is easy to catch foreign speaker (even writting), but... is the same for us foreign speakers?

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Well, self explanatory.

In Spanish, it's easy to catch foreign speakers (even when writing), but is the same true for us foreign speakers.?

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Catch my drift?

My job has taken me all over the world but every meeting and conference I have ever attended has been held primarily in English. I have so much admiration for those that are operating in a technical field in a non-native language. I can’t really imagine how difficult it must be. I see posting and discussing all things audio here on this forum in the same vain.  My hat is off to all those conversing in a second language. 

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.