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.
