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

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

Yeah, spelling can be pretty appalling in chat room postings.  The way I see it, though, it's damn near impossible to de-flack all the 'corrections' offered up by happy-go-lucky spell-check software.  Then there's the miserable keyboard choices available on today's laptop & other portable devices.

This just may be THE dumbest topic ever on A'Gon. Who cares if the poster is foreign ... si?

Si.

Yes, it is pretty easy to tell when someone is foreign to the English language. It's also pretty easy to tell when a native speaker of English is also foreign to the language. None of it really matters though. If you apply the principle of charity to someone's expression, you do your best to try and grasp the intent of the message regardless of how formally correct it is. And as others have said, irregular English patterns by non-native speakers are actually quite charming.