Sally_Hawn said:
Can French speakers tell I am not a Francophone if I say,
"Oh-la-la! Vous avez les belles fesses !"
There is almost no chance for a non-native speaker to pass for a native French speaker because of quite unique sonority
and intonation of French. English is also hard to pronounce for non-native but because there are so many accents and most English-speaking countries are immigrant countries, there is a margin of tolerance (which doesn't exist
at all in French). Only Parisian French is standard and any other accent is dismissed as "provincial" (read "dialectical" or just "wrong"). That is partly why French people make so much fun of Quebecois, which sounds like a very strong foreign accent (maybe stronger than a typical English accent in French).
French people are not tolerant when it comes to speaking their language, and that is partly why French has been replaced by English as a world language. French is and will remain and elitist language (also because it is more difficult and irregular than about any other languages).
Back to your sentence, you should say "Vous avez
de belles fesses" ("des" instead of "de" is also possible grammatically, but unnatural), although between young people it will be "tu as de belles fesses", or going more informal "t'as un beau cul !" (and don't even try "oulala" with this last one :sorry: ).