By their writing?
So, I saw this quiz on Facebook - Can you identify these 11 languages by their writing? Could be interesting, I thought. But then the questions were like this: "This is a West Semitic language of the Afroasiatic language family. What written language is this? Hebrew, Slovenian, Thai"
Wut? I don't even need to see the writing to answer that.
Or else this: What language is this? Comment allez-vous aujourd'hui French, Italian, Spanish
Guess what? No way to tell from the writing system!