I can tell any modern diesel from a petrol. There are plenty of posh motors around the area of London I go to uni in all with their silky smooth 3.0 turbo diesel engines (Audi A4/A8, BMW 5/7 series, Audi Q7 etc etc). I can always tell a diesel even if stationary or moving because stationary will have the unmistakable diesel clattering noise no matter how refined the engine is and when moving, whoosh of a turbo and slight clatter gives it away it's a turbo diesel. 6 cyl/8 cyl engines found in luxury motors nowadays have a different soundtrack, certainly one where I can distinguish.